Solar Questions

Michael Boxwell I've been asked solar questions by so many people over the past six years since the first edition of my book was published. In fact, one of the most fun aspects of having written my book has been meeting and talking to the countless enthusiasts who have bought the book and have implemented their own solar energy systems.

If you have any questions about solar, you can get in touch with me through my ask me a Question web page. Below you can see some of the questions I have been asked before about solar power, and my answers.

Solar Questions and Answers Archive:

what is the best direction for the solar system in lebanon?
is it to the south directly? or it should be different?
pleaze submitt me an clear answer to make my project.
with my regards
     ali ali hellani, February 22, 2012


I have a copy of your 2010 book and it is the best I have seen and is very helpful to someone starting out.
I am looking to purchase a travel trailer which runs at 30 amps max. I am wanting to set up a solar package to run the trailer. To start off with I have one 130 watt solar pannel, 30 amp controller, a 2500 watt inverter and one 12v battery at 125 amp hours. I am trying to estimate the number of additional pannels and battaries I will need to run the trailer at 30 amps for 8 - 10 hours. I could get by on less by not using the AC unit that pulls 20 amps. Any suppgestions would be helpful.
     Nick Gasaway, February 25, 2012

    Dear Nick

    Thanks for your comments about the book. It is much appreciated.

    By 30 amps, I assume you are talking about 30 amps at 12 volts, rather than a higher voltage? Based on this assumption, you're going to need somewhere between 400-600 amps of batteries so that you comfortably get 30 amps for ten hours without discharging the batteries too far.

    With regards to solar panels, it depends where you are based and how quickly you need to recharge the batteries. If you are going to be pulling 30 amps for 10 hours each day, that is a total drain of 3.6kWh per day and you're going to need a lot of panels to bring that back up again.

    To work out exactly how many, go and look at the solar irradiance table for your area (, and for the worst month that you need your system to work, divide the solar irradiance figure by 3.6 in order to work out how many kW of panels you need to install.

    If the figures work out too much (and I suspect they will), consider how you will be using your travel trailer. Will you be using it for a few days and then leave it parked up for a week or two? If so, consider more batteries so that you can store more energy, and then use fewer solar panels to charge them back up again.

    Frankly though, your best bet would be to look very carefully at what you are using your energy for. 30 amps per hour is a pretty heavy drain on your system. Reducing your energy usage will make this a far more cost effective solution.

Hello Michael,
Thanks for an excellent source of information- your 2010 release serves as a great practical reference for me.
I am interested in the relation between PV system size and payback period. Say for a given demand, location, and specific local power rates, how does the payback period change (if at all) as the PV system size increases. I 've used your solar calculator to estimate costs for demand loads of 100W, 500W, 5000W, and 25000W, and the payback periods (using my local power rates) seem to decrease slightly ranging from 6.3 years to 5.6years as we go up in system size. Obviously there are many more variables that come in to play as sizes/demand loads get into commercial levels. Also, can I assume your online calculator is targeted more towards the smaller DIY systems or do the calculated estimates cover a broader size range? Would you have any further insight into the current economics of scale with PV systems?
Barbados W.I
     Scott, February 28, 2012

    The online calculators are aimed at smaller installations, but a fair amount of work was done to ensure they work reasonably accurately for the bigger solutions as well.

    The payback really does vary based on local legislation. For instance, in the UK systems larger than 4kW in size are less cost effective than smaller systems due to the greater complexity in installing the grid tie, and the rates available for power from the electricity companies also drops with larger systems.

    I do not know what commercial rates you get for selling your electricity in Barbados, but in the main, the costs for installing a larger system work out slightly less per kW as the system gets larger as the associated costs for the controllers drop relative to the size of the system.

I am a student and I am doing a project on solar panels. I am doing research to find out if having a solar panel that does solar tracking and cleans itself is more efficient than a stationary structure.
So my question is if You cloud help me in anyway with this.
If you maybe have a solar tracking structure or circuit for it.
What motors to use etc.
I am doing electrical engineering, but I am expected to build the structure with bracket etc.
     Renaldo, March 1, 2012

    I do not have solar tracking unit plans available, although there are a number of web sites that either sell these or offer them free of charge.

    I would suggest you go and look at a website such as to see what you can find there. There are a number of people who have built some excellent DIY solar trackers and they may be able to offer you more assistance in this area.

    With regards to the relative performance of solar trackers, go and have a look at the NASA web site ( Create yourself an account and enter your latitude and longitude. Look at the different options and you can see how to find out the relative performance of a solar tracker compared to a fixed structure for your precise location.

    Good luck with your project.

Hello Michael. Thanks for the book. One of the earlier questions asked about shading and splitting the array into three strings, you replied that this would be better but I wondered if the shading on one string would reduce overall output to the Inverter anyway?
     David Ponton, March 2, 2012

    A 'string' refers to a series of solar panels connected in serial.

    If you have a single solar panel in a string in the shade, it pulls down the performance of the entire string. You can reduce this problem by having multiple serial strings connected in parallel, or you can almost entirely eradicate the problem using an intelligent controller that can accept multiple strings of panels.

Hello Micheal
i have 2 panels 80 watt of 12 v , can i hook them in series to a controller 24 v /12 v , then hook up the batteries in parallel to the same controller ? therefore i can use a 12 v to 220 v inverter since i am out of 24 v to 220 v ? is it possible , and what will happen if i do it

     Fadi Nassar, March 5, 2012

    No. If you connect the panels in series, you need to connect the batteries in series as well.

    The same is true if you connect the panels in parallel, you need to connect the batteries in parallel too.

Dear Michael
i am sorry to ask to many question but i am want to master this field since is my work .
just got a project , to install 2 km of solar post for the streets .
1- i did my research most solar post have 2 panels of 80 watt or 100 watt and 2 battery and work 24 v why? what is the difference if i use one panel of 160 watt and one battery of hight amps 12v system , witch is the best system for a solar post since maintenance is a hassle .
2- unfortunately in Africa they have the tendency to steal batteries , if i install the battery box under the panel it would be a hassle for maintenance , what do you think if i bury the battery underground ? it will effect the battery ? do you have any suggestion ?
3 - what do you think about steca solar control are they any good or witch do you suggest ?
thx again

     Fadi Nassar, March 6, 2012

    There is probably very little difference between running your system at 12 volts or 24 volts. I am guessing that it comes down to price: sometimes it is cheaper to buy two smaller solar panels rather than one big one, especially if you are bulk buying.

    I like the idea of burying the battery, but you would need to use a GEL battery for this purpose. These are entirely sealed and so long as they are protected from water (i.e. what happens when it rains?) this could be very effective.

    Steca Solar Controllers are my controllers of choice for small systems. They're cheap to buy, easy to set up and use and very reliable.

Hi Dude
My final year project is designing and testing of a portable photovoltaic system, but i am confused about how to calculate the angle at which the solar panel will draw more sunlight.
Also the solar panel i am using have a high light intensity of 1000 W/m2 and a low intensity of 800 W/m2, but i dont know which of the intensity i will use to calculate the power input on the solar panel since the sunlight in the UK is not very strong.
And finally, I will like to know how to calculate the efficiency of the solar panel and the amount of energy generated by the solar panel a day.
Thank you very much for your help and thanks for your handbook which i have jt ordered.
     David, March 6, 2012

    If you are using the solar insolation calculators on this site, you use the high light intensity figures.

    With regards to the angle, the optimum year-round angle for solar is 90-your longitude. This is the height of the sun at solar noon on the solar equinoxes (21st March and 21st September).

    During the depths of winter, the sun is 23.5 degrees lower and at the height of summer, the sun is 23.5 degrees higher. To get the maximum energy from the sun, you would need to adjust the solar panel angle each month.

    With regards to calculating the efficiency of a solar panel, the only real way of doing it is in a laboratory, measuring it against a fixed light source of a known intensity.

    To calculate how much energy is generated by the solar panel in a day, visit the Solar Insolation calculator on this site - - and enter your location. Multiply the wattage of the solar panel by the monthly solar irradiance figures to give you an estimate of the solar energy output from the panel on an average day of each month of the year.

Thanks for the my questions you answered earlier on. But please i will like to know what formular I can use to calculate the angles at which the solar panel can be positioned during the winter or in the summer (in the UK), in order to gain maximum power output.
Thank you.
     David, March 7, 2012

    The formula is this:

    Solar Equinox (21st March/21st September): 90°-local latitude

    The sun angle changes by 7½° from this angle each month. For example, on the 21st April - exactly one month after the equinox, the angle would be 90° - local latitude + 7½°.

    By the height of summer, this means the angle has reached 90° - local latitude + 23½°.

    In the winter, the sun is the same amount lower than the equinox date. On the 21st October, the angle is 90° - local latitude - 7½°. In November, 90° - local latitude - 15° and in December, 90° - local latitude - 23½°.

I am a solar designer. My client has a small multifaceted roof - the larger roof (X) (facing about 40deg west) can hold about 6 modules (1.3kW) and the smaller roof (Y)(facing 50 deg east) can hold only two modules (0.4kW). The sizes are too small to work with a dual channel MPPT inverter. My understanding is if I join all 8 modules in a single string, then the whole array will have a lower output than the sum of the theoretical output of the 6 modules in plane X and the output of the two modules in plane Y. How do I estimate the combined output? Thanks
     ANTHONY, March 9, 2012

    I can't give exact figures, because there is a huge disparity between different makes of solar panel. In some cases, having one panel in partial shade can reduce the output by 80%, whereas others make a very much smaller difference. I'm told that the latest amorphous panels have a negligible drop, although to be fair I haven't tried this myself. I do know that amorphous panels generally work well in shaded conditions.

    Some manufacturers actually provide the shade information themselves, but most of the time I've had to find out by rigging up a panel with a multimeter and then seeing what the effect is of partial shading myself.

    If you have a chance to 'play' with the solar panels yourself, try connecting them in series, then try connecting them up in strings of two, connecting each string together in parallel. I think you would be surprised how much better the parallel solution works when shading is taken into account.

My daughter came home with 3 very small flexible solar strips. Her teacher wants us to build a bird house and hook a light to the house. The strips have no place to hook wires to, so I thought I would ask how do we hook up the light to the small solar strips? Also what type of light should we use with only 3 small strips.
Thank you for your time,
Jeff Morgan
     Jeff Morgan, March 14, 2012

    Sounds a strange project! You need to measure the voltage from the solar strips - I suspect they're 2 volts each.

    You can then either connect the strips in parallel to give a bigger current at 2 volts, or connect them in series to increase the voltage to 6 volts.

    If you want the solar strips to power the lights directly, you can connect a bulb directly to the solar strips. However, this isn't particularly useful because if the sun is shining you don't need a light bulb!

    Alternatively, connect the solar strips to a battery and then wire up a light bulb to the battery with a switch.

    If it were me, I would be tempted to wire the strips in parallel, using them to charge one 'AA' or 'C' rechargeable battery. Put a switch between the solar strips and the battery so you can switch off charging if the battery gets warm (which would indicate it is fully charged). I would then use a 1.5 volt torch bulb and wire this up to the battery, again with a switch in place so that the light can be switched on and off.

Dear Nick, Just read your book. I am about to install a grid tied solar array on my flat roof. I am interested in a manually adjustable mount for fairly large panels. Thank you for your help.
     Rory Sanders, March 15, 2012

    Speak to a couple of local solar panel suppliers to see what they recommend. There are number of different mountings available that will do what you want, and they are not expensive.

    Have a look at the ones on this website to give you an idea of what you can get:,-adjustable-pv-stand-%252d-5wp-to-150wp.html

Interesting work. What is your source of data for the online calculator?
     Carlos E. Del Castillo, March 17, 2012

    NASA publish the information and their calculations on their own website.

How do I work out the optimal angle for my solar panels for maximum total power generation throughout a year? I used your angle calculator for month by month, but I only have the option of keeping them at a fixed angle all year round. My priority is generate as much power as possible. I am based near Gloucester, United Kingdom.
Many thanks
     Adam, March 20, 2012

    For maximum total power generation, you angle your panels at the March setting. If you look at the solar irradiance calculator, you can see options at the bottom of the screen for choosing a different setting and see the difference in power generation depending on the angle you choose.

Hi, I was wondering what would happen if I have multiple strings of 235W panels and strings of 230W panels going into one inverter? Will the 235W strings act like the 230W strings?
Thank you
     Edgar Lim, March 22, 2012

    It depends on the inverter. Some inverters allow you to connect multiple strings together, taking a different feed from each string. Others tie everything together at the input.

    With the former, the 235w panels will act as 235w panels and the 230w panels will act as 230w panels. With the latter, you will get some compromise between the 235 and 230 watt panels, but because the panels are so close in rated output, you probably wouldn't notice the difference.

Dear Sir,
I want to charge 12volt 150AHr battery by solar pannel.
so i want to know that how much watt Solar pannel is required for best charging .
I am a student & want to for temple light.
     dheerendra, March 23, 2012

    It depends on various factors - specifically, how many lights, the wattage and for how long each day you need the lights to work. It also depends on where abouts you live in the world and the angle that you can mount your solar panel at.

    For an accurate answer, go here and fill out the questionnaire:

    You will then be emailed a report telling you what you will need for your project.

you appear on the Reva G-Wiz club site talking of a solar panel for the Wiz. I could'nt join the club as my hotmail address was disallowed due to spamming.... hence I'm trying you here.
I've just bought a G-Wiz on ebay and am trying to put together a solar roof for it, I stumbled upon you doing google searches.
My thinking was to bond an array of 4 x 12v flexible monocrystalline solar panels to the roof and bonnet and wire them in series and connect them to the battery array. If I managed to exceed a total of 100w of panels I'd need a 48v charge controller? I've seen 30w panels so I could possibly end up with 120w of power.
Have you got anywhere with your solar roof project? Do you know of any off the shelf 48v charge controllers... I've only seen ones aimed at the domestic set ups that would handle 40amps and are a touch overkill and pricey! When I searched for solar roofs for John Deere e-Gators which I've worked with before and have a 48v Curtis system using an array of Trojan T105's or T145's I came across a californian web page where they'd done one for the grounds dept of Lewis uni and it had a quite positive write up. another californian company does a very sexy pv roof for the prius at prius prices! On my budget the G-Wiz is my entry level for EV's and I'd love to optimise it as much as possible, any feedback on this subject/progress on your project would be avidly digested!
     Matthew Baker, April 10, 2012

    Because the G-Wiz has 9.6kWh of battery storage, you don't actually need a charge controller as the solar panels will not produce enough energy to damage the batteries through overcharging. You should be able to just connect the panels in series to build a 48 volt array and then just connect this to the battery (with a blocking diode so the solar panels do not discharge the main battery at night).

    I did actually design a 48 volt, 80 watt solar roof panel for the G-Wiz, designed to mount on the roof of the car. Unfortunately, the production costs did not make it viable to proceed with the project.

    The benefits of putting solar panels onto a G-Wiz were two-fold: by trickle charging the batteries during the winter day, the battery temperature could be increased, thereby extending the range by as much as 20%. During the summer, the panel added around 10% additional range to the car.

Dear Michael,
I sent the following message to you a couple weeks ago as a reply to your reply to my first query. Perhaps that is not getting my message to you. I will try again thru your site:
Thank you for your recent reply, and your encouragement on our project to initiate electrical lighting at a feeding center for orphan children in Malawi, Africa this June. Your book has been our primary guidance textbook in undertaking this project. As I mentioned, we are planning to use two Kyocera KC50T panels, wired in series, to supply 100W power. We have ordered the panels, as well as a Morningstar Sunsaver MPPT-15L controller. We are planning to employ two 50 amp-hr batteries, configured in series, to generate a 24 volt supply. We chose a voltage of 24 volts to keep wire size to no larger than 10 AWG. For this first installation we will carry all of the components, except the batteries, with us from the US. Smaller wire size will be much easier from all standpoints.
Our objective has been to provide lighting, via overhead LED lights, for a remote children’s feeding center to facilitate evening classes, reading, and other activities. As we have begun to investigate availability of LED light bulbs, it appears that 12 volt bulbs are cheaper and easier to source than 24 volts. I have considered wiring the lights in two circuits, one with half of the 12 volt LED’s in a circuit connected across one 12 v battery, and a second circuit with the other half of the bulbs connected across the other battery.
However, in your book you have highlighted the importance of draining batteries, connected in series, in equal fashion. You stated that if equal discharging or charging is not maintained, the batteries will not all recharge equivalently to their full capacity, and battery life will suffer. Is is feasible to wire separate circuits as I have described, one to a battery, even though those batteries are wired in series? I suppose another option might be to wire 12v LED’s in pairs, across 24 volt wiring. I have no basis to know the best way to handle this.
In addition, as I’m certain you have experienced, all projects “evolve”. We have just been informed by our sponsor in Malawi that their long term desire, after we install our first solar generation station with overhead lights and a cell phone charger, is to expand it’s generation capacity sufficiently to become a kiosk recharging station for battery powered lanterns. The LED lanterns would be purchased and then leased to villagers who would return them daily to the solar center for recharging. My understanding, however, is that these lanterns operate at 6 volts! Can we handle that? I suppose we could charge from the solar panels a series(4) of 6 volt batteries in series and draw 6v power from each battery junction, or connect 4 lantern batteries in series across the 24 volt solar battery pair and charge them all at once.
Michael, have you encountered situations such as these? Would you provide us guidance upon how to charge a solar battery at one voltage, with minimizing wire size in mind, but then distributing power at other, lower voltages? Is there a simple way to do this? Is there a controller that will monitor batteries wired in series, but connected to independent circuits? I would very much appreciate your assistance.
Paul Sigmund
     Paul Sigmund, April 11, 2012

    The way to handle these different voltages is to use a DC-DC Inverter so that you can get a different output voltage. Thankfully these are easy to get hold of: most trucks work at 24 volts and you can buy an adapter to drop the voltage to 6 volt or 12 volts in order to charge cellphones or run 12 volt accessories.

    These units are often known as 'voltage converters' or 'DC-DC converters'. A quick search on Google should find what you are looking for.

Hello Michael.
Is your book available in an e-book format? I need it ASAP and can not wait for delivery!
     Corrie Pistorius, April 12, 2012

    You can buy the book in Kindle format from Amazon. A PDF version and EPUB version is currently being worked on and will be available in May.

Can you explain fully how the feed in tarrif works, are you paid the 41p for each generated kwh of energy even if you dont use it? And is all unused energy fed back onto the grid? ie: if I generate 2kwh but only use 1 kwh am I paid for 2 at 41p plus 3p extra for the 1 I didn't use? Also on a more technical side, how do the two supplies, 1 from the roof and 1 from the grid, change over when either one is used to supply the board or are the simutaneously feeding the board with no change over, in other words wiil items like computers loose supply for a moment when switch from grid to roof?
     Paul Carr, April 12, 2012

    Yes, the UK feed in tariff pays you for electricity generated, even if you use it yourself. This is, in effect, the government subsidy that they are using in order to encourage more people to take up solar.

    With regards to the switch-over from roof to grid, the inverters job is to provide power that perfectly synchronises with the grid so that when the system switches from one to another there is no momentary loss of power.

Hi there. I have a 125watt solar panel, and I am wanting to get the most out of this. I am thinking of purchasing another one, but I am quite new to this and still doing my research. I am wondering if it is possible to have too many batteries in a solar system? Also, how do you know what size inverter you need? I want to be able to run as much as I can off this panel and was just wondering if there are "capacities" when it comes to batteries and an inverter sizes. Or can I just get a large amount of batteries, and a huge inverter? Then adding panels as I can afford too?
     dan, April 19, 2012

    It is possible to have too many batteries for your solar array: a battery will typically discharge by around 2-3% each day, so if your solar panel is too small, it won't even maintain the charge in the battery, let alone top it up! The largest battery pack you could maintain with your 125 watt solar panel is around 4.1kWh of battery pack.

    Of course, if you are using lots of power from the pack, that solar power is not going to be able to charge it up particularly quickly, but if you want to start small and then build up, that is okay.

    Inverter sizes are calculated based on your power demand. There are two ratings on an inverter: continuous output and peak output, both shown in watts. For instance, an inverter could have a continuous output of 1000 watts (1kW) and a peak output of 2000 watts (2kW). This means that it could maintain a constant 1kW power supply, but for very short periods of time - typically a few seconds - could provide additional power. For instance, some appliances like a fridge use a lot of power for a few seconds when starting up before dropping back down to a much lower current.

    It is usually recommended that you buy an inverter that can provide at least 30% more power than you actually need. So, for instance, if you need 1kW of constant power, you should buy an inverter that can provide you with at least 1.3kW of continuous power.

In the Online Calculator the units for solar irradiance is listed as kWh/m^2/day. How do you use this number? Do you ignore "/day" and use the value as kWh/m^2 for that specific month? I would appreciate if you could clarify this for me.
     Sunny Ishikawa, April 20, 2012

    The iradiance shows how much energy you get over the period of a single day, from sunrise to sunset.

    For instance, if for a specific month you have 5 kWh/m^2/day, that would mean the strength of the sunshine provides 5 kWh of power across a square meter of the surface of the earth, spread over the period of the whole day.

    If your solar system is 15% efficient, you could then expect to get an average 15% of 5kWh of energy per day from your system.

     RAUL STORY, April 21, 2012

    The book is already available as a Kindle book and will be available in EPUB format in early June.

With your experience could you give me an indication of what output you would expect to lose during a hot 25-30c day as compared to a cool day 10-15c. I am asking as although I can find reference to output loss during hot weather I cannot fined any indication of what you might expect to lose, ie 5% 10% 20% etc.
Many thanks
     Gary Hancock, April 24, 2012

    Pages 41-43 of my book give this information in detail, but briefly, what you are looking for is a 'temperature coefficient of power' rating on your solar panels.

    When solar panels are given a wattage rating in a laboratory, they are tested at 25°c against a 1,000w/m¹ light source. At a cooler temperature, the solar panel will generate more power. At a warmer temperature, the solar panel will generate less power.

    The 'temperature coefficient of power' rating on a solar panel shows the percentage of power reduction per 1°c increase in temperature.

    This does vary from one manufacturer to another, but will typically be in the region of 0.5%. This means that for every 1°c increase in temperature above 25°c, you will lose 0.5% efficiency of your solar array. So on a hot day with temperatures around 35°c, you would lose around 5% of your rated power.

    On a cooler day, efficiency increases by a similar amount. So on a cooler day, around 15°c, you would gain around 5% of your rated power.

    Hope this helps.

i have a inverter 300w and 12v 100ah battery.i want a solar panel to charge the battery .
Which type of solar panel have to use for charge the battery?what will be the cost?
     jensonjohn, May 1, 2012

    It depends on how quickly you want to recharge the battery and where you are based.

    In order to come up with some proper figures, you need to work out what power you are using, measured in watts, and how many hours a day you need the system to be providing this power.

    Once you have this information, you can use the Solar Project Analysis tool on this website to work out what size solar array you need and come up with some reasonably accurate costings.

i want a solar panel to charge a 12v 100ah battery.which type of solar panel can we use?detail & indian price of that panel?
     jensonjohn, May 1, 2012

    It depends on how quickly you want to recharge the battery and where you are based.

    In order to come up with some proper figures, you need to work out what power you are using, measured in watts, and how many hours a day you need the system to be providing this power.

    Once you have this information, you can use the Solar Project Analysis tool on this website to work out what size solar array you need and come up with some reasonably accurate costings.

    The Solar Project Analysis tool can provide you the costings in rupees.

I tried using the project calculator and received an error.
I would like to do solar powered air conditioner. Do you have any suggestions? Currently I own one that uses 1200 watts and 10 amps. Do you know of any that are specialized for solar?
     Spencer, May 8, 2012

    I apologize about the error that you received. We have had one or two issues recently. Hopefully we have resolved these now.

    Solar air conditioning is possible, but not using off-the-shelf components as these are too power hungry. Solar powered air conditioning units are available from a number of suppliers, including Hitachi. Here are some links that you may want to look up:

I am living in london middlesex. how much solar pv generates units in 1 day on south facing per panel genearation if you can tell me in detail. please explain me in detail thanks.
     hima, May 10, 2012

    The amount of power you will generate will vary throughout the year, based on the seasons and the weather. It also varies based on the exact angle of the roof.

    However, within those confines, it is possible to come up with some reasonably accurate estimates of power generation capacity on a month-by-month basis using the Solar Irradiance calculator on this site:

    Enter your location. For each month, you will see an irradiance figure. Multiply this by the wattage of the solar panel(s) you are planning to use and this will give you an approximate figure for how much power you could expect to generate each day.

    You can then experiment with the calculator to see how the performance varies depending on the angle of the solar panels.

Hi Michael...thanks for your great book. I do have a question though. On page 82 (2010 edition) regarding wiring the solar array...why wouldn't each string series connection of (2) panels 12v 10w end up being 24v 10w in lieu of 24v 20w? Hence the total configuration after the parallel connection of the (2) stings make it 24v 20w in lieu of 24v 40w? thanks...
     Bill Saar, May 14, 2012

    I suspect you are confusing power (watts) with current (amps), which is a very easy mistake to make (I made that same mistake myself a dozen times when I was starting out!)

    When you put two solar panels together, you are doubling the power of the system (which is measured in watts). So if you connect two 10w panels together, you end up with a 20w solar array.

    When you connect two panels in series, you also double the voltage (i.e. 12v + 12v = 24v), which means the current remains the same as if you had a single panel. If you connect two panels in parallel, the voltage remains the same as the single panel (i.e. 12v) but your current doubles.

    Re-read the chapter entitled 'A BRIEF INTRODUCTION TO ELECTRICITY' starting on page 11. Hopefully that will then make a bit more sense!

Hi I recently purchased your book and plan to set up my own solar power for my home. My fquestion after reading your book is you suggest an inverter that produces pure sine wave, but what is the difference between pure and modified? Is there a big difference and should I be vigilant about only using a pure sine wave inverter? Thanks Steve
     Stephen Brandenburg, May 14, 2012

    There is a good introduction to the different types of inverters on this site here:

    Basically, pure sine wave inverters are a lot more reliable. They'll work with pretty much any electrical system, where as some electronic equipment will not work well with modified sine wave inverters.

    If you are planning to run electronic equipment such as computers, TVs or music systems, you should use a pure sine wave inverter.

Michael, I just ordered your book and have really enjoy your site!
I hope you have a moment to provide a novice with some advice on a system upgrade. I've been using an el-cheapo 60 watt amorphous panel system I bought at a box store for basic lighting and a few other things as an off-grid battery system. It is working fine, but it came with a very basic charge controller and inverter which are OK for my current needs. I plan to upgrade the system as the budget allows, eventually switching to high end panels, but right now I'm considering upgrading to a basic MPPT charge controller (BZ PRODUCTS MPPT250HV). This unit has a higher voltage input option (100 volts max) and a PV charge rating of 25 amps continuous. My question is, should I rewire my existing 4 panels (15W x 1amp each @ 16 - 24 OCV) from parallel to series or a combination to take maximum advantage of the MPPT until I upgrade the array? My other concern is the specifications call for a minimum input of 68 watts so I may not gain much efficiency-wise right now, but at a mere $140 USD, I feel it's still a worthy upgrade over the cheesy controller I currently have. Does this sound like a feasible plan considering I do not have room for a large pv array at this location?
Best Regards, Mike

     Mike Lazyo, May 14, 2012

    I've used a very similar system myself on a project that needed a very low cost solution and I was very impressed with the results. You are absolutely right about the controller, however. It's cheap and cheerful, but you will most definitely improve your performance by replacing it with an MPPT controller and will probably get a 20% improvement in capacity as a result.

    I do not know the BZ Products units personally. I would recommend speaking to your supplier to get his opinion, but I have had a quick read through the installation manual on the BZ Products website and it does appear to be suitable.

    It is up to you as to whether you rewire your array to work in series rather than in parallel, but remember you will also need to rewire your batteries to store 48 volts rather than 12 volts. So long as your cable runs are very short, the efficiency gains will be very small, so you are probably best leaving everything at 12 volts for the moment.

I have started working on a project of 5MW PV which will be a grid-connected system. according to the project methodology, please tell me how i could calculate expected total energy generation(MWh)for each month from monthly solar irradiation(KWh/m2/day)data.
Somesh Shah
     Somesh Shah, May 20, 2012

    Simply multiply the daily figure (kWh/m2/day) by the number of days in the month.

Dear Michael,
I love your book. Practical and strict to the point.
I have a question regarding the solar irradiance unit.
The unit is in Kwh/m2/day or in hours of sunlight.
Thank you
     Gabriel Lailo, May 20, 2012

    Thank you for your comments about the book. The unit is kWh/m2/day, which is not the same as the hours of sunlight.

    In effect the kWh/m2/day figure shows the equivalent number of hours of noon-day sun, spread across the whole day.

Just bought your book through and I'm ready to start a Stand Alone and Grid Failover system in an addition to my home I am starting soon. I've heard about the Power Shingle Dow Chemical is manufacturing to be avaiable first of 2012. I'm timing the construction of this room to be roofed at the time these shingles are sold. What's your take of these shingles?
     George, May 22, 2012

    I have not seen these shingles myself, but I have heard good things about them from the people who have seen them.

    Of course, nobody has actually used them in a real project, so it is not possible to say how good they are going to be in practice yet. However, Dow Chemical is a reputable company and the technology behind the shingles is tried and tested.

    I am intending to get a small number of these shingles next year to carry out my own testing on them as I have had several readers get in touch with me about them. From what I have seen, they look a good product and I am looking forward to having some to work with.

I'm from Gujarat, India. I would like to know percentage imporvement in generation with seasonal tracking only with four angle shift in a year. I came to know in India because of its position on earth, less percentage gain would be achived vis a vis other europen countries.
     falgun patel, May 23, 2012

    Thank you for your message. You can find the information on the Solar Electricity Handbook website by looking at the Solar Irradiance Calculator - - and selecting the angles for winter, summer and spring/autumn.

    Alternatively, if you want greater detail, I would recommend you visit the NASA Solar website. You will need to create your own account, but then you can select the information you require:

Hello Michael,Would you mind giving me your advise,as to which manufacture of solar panels and inverters in your opinion is the best currently available in terms of efficiency etc. My house is south west facing 150degrees and we live nr Bridgwater /Somerset. Is there a league table of solar panels that is indepandently compiled,rather than tables that I have found on the internet that seem biased/ spnsored by panel manufactures. Your comments would be very much appreciated as we want to put panels on our roof but want to do it right first time. Regards Andy & Barbara Jackson
     Andy Jackson, May 28, 2012

    There are some really good solar panel manufacturers around now and the days of second-rate solar panels from shoddy suppliers thankfully seems to be in the past.

    The very best panels, in my opinion, come from Kyocera and Sanyo. The build quality is good and I have had some excellent results from these panels over several years of use. However, I have also had pretty reasonable results from much cheaper Chinese manufactured product: the performance is very slightly lower, but the cost is significantly lower as well. I recently inspected a system I put in five years ago using cheap Chinese solar panels and they are still performing very well.

Hi Michael,
Is there a roof frame system that allows you to tilt the frame and panels to the optimum angle to maximise the sun rays on a month to month basis to get the most efficency from your panels.
Andy & Barbara Jackson
     Andy Jackson, May 28, 2011

    There are a couple of different systems, but they do tend to be quite expensive. They are also fairly heavy and difficult to install.

    As a consequence, it is probably cheaper and easier to buy a couple of extra panels rather than to pay more for a heavy and cumbersome frame.

Ihave 10&15w panels,26ah battery and 15amp controler.These panels do not charge the battery as required.What should i do or buy so that i can charge my battery in order to produce AC 220V with my 220w inverter to be used for 5hr daily to my 14 inch colour tv?
     gaston, May 29, 2011

    Use the Solar Project Analysis tool to come up with an accurate estimation to do what you need:

Dear sir,
I just order your handbook 2010 and reading it.To begin I don't have any idea of solar energie.I live in Aruba.So I decide to read your book for more know how of solar panels.I would like to know can I use this for airco's at home.I have four or where can I get more informacion for my project.

     Ronny Noguera, June 2, 2011

    You can get solar powered air conditioning systems, but they are not cheap. There is a useful wikipedia article on solar air conditioning which is worth a look:

    There are a couple of companies who now produce DC-powered air conditioning units that are specifically designed to work with solar power:

    However, air conditioning systems are very power hungry, so you will end up having to spend a lot of money on solar panels if you want your system to be 100% solar powered.

    Hitachi have recently launched a solar powered air conditioning system using the concept of heat exchange with solar thermal technology. I have not seen one myself, but the technology certainly sounds interesting and - theoretically at least - should be more efficient and cost effective than a pure solar-PV based system:

Need a Glare study for the roof mount we are installing for the building department, where can I get one? I was told there is a calculator online somewhere, spent hours looking for it, nothing. PPlease help
     Jane, June 2, 2011

    As far as I am aware, there is no online glare study system available. I have had a chat with my software designer and a local architect. Unfortunately, we've come to the conclusion that producing one would be almost impossible because ambient lighting can vary so much for so many different reasons, coming up with a pure web-based system that is accurate enough and reliable enough would not be possible.

    I would suggest you need to speak to your solar supplier about it. You will probably have to pay them to provide a proper survey. Sorry.

I read your book - Thanks, it is a good book. Right now I have 120W solar panel running 12v dc attic fans.
My main question is the drop in efficiency when adding a charge controller ( non mppt) and deep cycle marine batteries. Do you have data on mppt and non mppt charge controllers on the loss of power out compared to solar power in?
     bill brodhead, June 6, 2011

    The efficiency levels do vary, but typically you will get 70-75% efficiency out of a non-MPPT controller and 90-95% out of an MPPT controller.

    However, efficiency of non-MPPT controllers will often be higher on overcast days, or at the start and end of the day.

    Thankfully, the cost of MPPT controllers are getting much cheaper to buy than of old, to the point that many suppliers are now phasing out the non-MPPT controllers completely.

    Battery efficiency has more variables, including temperature and how quickly you charge and discharge them. To get the best out of deep cycle marine batteries you want to keep them between 25-35 degrees centigrade (77-35 degrees fahrenheit) and you want to ensure that you charge and discharge them at their C20 rate or below (i.e. it takes at least twenty hours to charge them up from flat and twenty hours to discharge them from full to flat).

    You also want to keep the number of times you 'cycle' the battery - i.e. completely discharge and recharge it - as low as possible. Ideally, you should not discharge your batteries below 50% and preferably keep them up closer to 80% state of charge for the best efficiency and longest life.

    Look after your batteries and they will always give you between 80%-95% of the power you put into them. Work them hard and within a couple of years they could be down to as low as 60% efficiency.

Hi Sir
I'm on the team to build a Solar race car and want to know how cloud cover affects the power output of the arrays we will be using.
For example the array delivers 30.7V(Voc) at 7.2A(Isc) maximum solar irradiance(so I've been told), was wondering how cloud cover would affect this?
Thanks in Advance
Chad Miller
     Chad Miller, June 9, 2011

    Cloud cover obviously makes a big difference, but how much depends on a number of factors - both environmental (i.e. how cloudy it is) and the performance of the solar cells themselves.

    You can buy a cheap solar irradiance monitor that will help you with the environmental side, and the manufacturer should be able to tell you more about the performance of their cells.

    For a rough rule of thumb, assume around 20-25% efficiency in cloudy conditions.

I have 110v 22.6 amp water pump, I want install a Solar Panel Kit, What is the best solar equipment for that?
     Frank A, June 14, 2011

    That sounds like a very powerful water pump! By my calculations that suggests that your water pump is using 2½ kilowatts of energy per hour.

    Quite frankly, that is going to cost you an awful lot of money to convert to a solar system. Without knowing where you live it is not possible for me to tell you how big your solar PV system needs to be, but if you fill in the solar project analysis questionnaire ( the system will automatically e-mail you a report that will tell you what you need to know.

how much solar cells do i require to charge
500 volt lei-ion battery.with time range of 3 to 4 hours.
     jagadeesh, June 14, 2011

    I need to know more information to be able to give you an answer.

    I would suggest you fill in the Solar Project Analysis questionnaire in order to get a feel for the size of your array:

i am currently doing a project in my college . i am having a 48 v and 130amp battery which i want to charge through both solar as well as ac main . can you please tell me about how to charge my battery . i do not want 100% of my battery through solar . i just want some % when there is no current . so please suggest me about the charge controller which will help to solve this problem.
     abhishek, June 14, 2011

    You'll need multiple solar panels connected in series to produce a 48-volt system - most solar panels are either 12-volt or 24-volt units. You will then need to connect these panels to a controller which is then used to charge up the batteries.

    Because you only want to trickle charge the battery, the size of the solar panels is not so important, so you can go for whatever you can afford. Every 48 watts of power that you produce at 48 volts equals one amp of power, so based on that and used in conjunction with the solar irradiance calculator on my site ( you can calculate roughly how much energy your solar panels would feed into your batteries on a daily basis.

I am purchasing an electric gate that uses two 12volt batteries of 7.2 amp hours to power 2 12volt electric motors. The batteries are charged by a 20 watt solar panel which ideally I would like to locate 80 feet away for best exposure of the panel to the southern sky. The gate is in a heavily wooded area. My other soution is to remove a few not so large trees so the solar panel has access to the sky.
Thanking you in advance
     Doug, June 18, 2011

    Whilst you can locate the solar panel further away, you will need a much beefier cable to overcome the resistance from the distance you are planning to run it.

    Instead of cutting down a few trees, why not get a bigger solar panel? Depending on the amount of shade you have will depend on how big your solar panel needs to be, but it would almost certainly be cheaper than cutting down some trees.

    You can buy 20 watt solar panels for around £60/$80, which would allow you to double up on your solar power for reasonable amounts of money.

I am a student and I am doing a project on solar panels.
I have to design power plant 5kw,How design it ?

     nasim, June 18, 2011

    You do not say very much about your project. Is your 5kW power plant going to provide power for an off-grid system, or is it to provide a source of power for the utility grid?

    If it is the former, you will need to store your energy in a battery bank. If it is the latter, you will need to have a grid-tie inverter to convert your power and feed it into the grid.

    Off-grid systems are usually run at a lower voltage than grid systems: typically 12, 24 or 48-volts, powering a bank of deep cycle industrial batteries, with the batteries managed using a solar controller. You take power from the system through the batteries, typically using an inverter to convert the voltage to a grid level voltage.

    Grid tie systems often run at several hundred volts, with the solar panels connected together in series in order to provide as high a voltage as possible. Power is fed directly into the utility grid from the inverter. There is no capacity for storing energy for night time use.

I have been trying to locate a manufacturer or a Wholesale Supplier for the Whole Solar Electricity System for quite sometime but failed so far.
V hav Strong Sun Light all year round and a great shortage of electric power and now the Govt of Pakistan has allowed import of the Solar Systems on Nominal Taxes as such
it has become worthwhile to import and install w/o any hassel.
v need at least 10 to 15 Kw systems off grid and on grid 220V single phase
     Shamim Ahmed, June 21, 2011

    You need to speak to your local electricity company about any limitations on single phase with that size of system. In some countries you are limited to around 3.5-4kW on a single phase, but I do not know if there are any limits in Pakistan.

    There are a number of solar manufacturers in China, and even the big brands such as Kyocera and Sanyo have manufacturing in China, so I would recommend that you speak with them.

    You may also find some useful contacts in this list:>ype=country_PK&xtype=ntype

Dear Michael,
We live in Northern CA at the snowline 5000 feet. We get a minor amount of snow but enough to stick to the satelite dish and knock out the signal, requiring clean off. We would like to add solar panels to our home as we have an large family and enormous utility bills each month. We lost power several times this year as long as 8 consecutive days. What are your suggestions for a system in our situation. Our home is about 3100 sq ft and there are 10 people full-time and up to 20 people during family gatherings.
     Matt and Carmi, June 21, 2011

    I would recommend a hybrid system for you where you are still reliant on some grid power, but use solar for as much as you can afford, configured as a 'grid fallback' system.

    In this scenario, you would run some of your household circuits on a solar system, running through batteries so you have energy throughout the day and night. If you flatten your batteries, the system then switches entirely over to the grid to charge them up again.

    During the winter, when I guess power cuts are more likely, you could switch this across to a 'grid failover' system so that your house is powered almost entirely from the grid, switching over to solar if there is a power cut. That way, you have the greatest resilience to power failure.

    Because of the size of your home and family, however, it is probably unrealistic to expect to be able to run all your electrical components purely from solar. Whilst it may well be technically possible, I suspect the costs could very quickly run out of control. Instead, I would recommend you run a few circuits through this system, providing essential services (lighting, refrigerator, heating timers and pumps) plus a few lower-power luxuries.

    Do an energy audit on your home and see how much electricity you use on a daily basis. Then work out how much electricity you use on essentials (thankfully, that figure is much, much lower!) and then you can cost up your system from there.

I have the book on order. The online info is great!!
When adjusting panels for optimal tilt angle by the month;one would assume that January adjustment is made January 1; but with all the focus on the 21st, can you elaborate on the start and change dates to use when making monthly adjustments? On what date would the January tilt angle be set? Please advise on the exact date to make these monthly adjustments.

     Dan Zimmer, June 22, 2011

    You don't have to be so accurate as to the exact day, but actually the best day to move the panels is on the 7th or 8th of each month. But 7-10 days either side of these dates makes negligible difference.

what is defination of NOCT? what will affect on solar modules? if i will not count NOCT what will happen to my panels para meters?
     dhaval, June 22, 2011

    NOCT stands for 'Nominal Operating Cell Temperature' and it is the temperature at which the solar panel provides its optimal performance.

    The standard in the industry is to measure solar panels at 25 degrees celcius, which is 77 degrees fahrenheit. At higher temperatures, the efficiency of the panels drops whilst at lower temperatures, the efficiency of the panels increase.

    The amount by which the efficiency changes depends on another figure - the temperature coefficient of power rating. This is provided by solar manufacturers and shown as a percentage of total power reduction per 1 degree celcius increase in temperature.

    As a general rule of thumb, this figure will be in the region of 0.5%. This means that for every 1 degree increase in temperature, you will lose 0.5% efficiency from your solar array. I cover this in my chapter on Calculating Solar Energy in my book.

    There is a good introduction to NOCT, along with an online calculator, shown here:

I have just bought the Kindle version of your book and am looking forward to reading it.
I am trying to work out whether it might be worth installing a grid tied solar system on a house with less than ideal positioning, i.e. roof facing more west than south (approx 260 degrees) and with a roof slope (from horizontal) of about 25 degrees
I have obtained a sun chart for my location in Cheltenham and therefore now know the direction and height of the sun at various times of the day throughout the year.
Can I now calculate the losses from optimum performance simply by trigonometry, i.e. if the sun will be striking the panel at 30 degrees from a direct, vertical angle, it will be receiving about 87% energy and at an angle of 60 degrees it will receive 50% ?
Presumably I also need to calculate separately the losses due to both vertical and horizontal misaligment, i.e. if the panel was tilted away from the ideal position by 30 degrees vertically and 30 degrees horizontally, it would be receiving about 76% of available power (87% x 87%)
Once I get this straight in my head, I can start to worry about the local irradiance calculations !!
I hope I have explained this clearly and will look forward to hearing from you - many thanks
Brian Heather
     Brian Heather, June 22, 2011

    The calculation is a little more complicated than that, because the most powerful sun energy comes at solar noon when the sun is due south. You will also end up with more shade because of your angle.

    It is possible to calculate the differences in power and I am currently working on a version of my online calculators that will allow you to do precisely that. They should be on this site within a month.

I have just emigrated to New Zealand. CN PV (Solar power SA ) , Chinese based, HQ Dongying , Shandong , China , are being proposed. Have you any knowledge of these.
Many thanks
Ian Taylor
     Ian H Taylor, June 23, 2011

    I do not have any experience with those panels myself and asking around, I haven't found anyone else in the UK who has used them either.

    However, most solar panels are now produced to a much better standard than they were a few years ago and my experience of the cheaper Chinese panels is that they are now almost as good as the mainstream brands - and significantly cheaper as well.

    You do need to be able to physically check each panel however, when they arrive, and reject any that you are not happy with. My experience has been good, and the engineering quality of most Chinese products is now very good indeed, but I have heard of other people who have had problems because of the build quality - specifically, poorly fitting frames that allow water ingress.

This question is really urgent. Kindly help me out. I am working on a solar simulator with a beam output of 51mmx51mm. But I need to measure the output of 100x100mm solar panel. When i placed the solar panel under the beam. It seems it illuminates the whole area. But in actual the beam output is not diffracting it is collimated beam. If i adjust the beam output to 1 sun. Then what would be the irradiance on solar panel which is approximately double the size of beam.
     rabia, June 24, 2011

    I have not carried out these sorts of experiments myself, so it is difficult to give you an accurate answer. There are also a lot of environmental aspects that will make a difference: most specifically what is the ambient lighting in your test area and what is the temperature?

    Assuming that there is no ambient light and the temperature of the panel is 25 degrees celcius (the Nominal Operating Cell Temperature that all solar panels are tested at), I would expect you should get one quarter of the official output of the panel, because you are only providing 1 sun output on a quarter of the total surface area of the solar panel.

how to generate electricity through solar
     udoye charles, June 24, 2011

    If you want a brief introduction to how solar works and how you can use it, I'd recommend reading the first couple of chapters of my book. You can read the first few chapters for free here:

Just picked up the 2011 edition from Amazon. Great book!
Question: I looking for a charger/inverter to install for a 3kw solar array off-grid system. (seems like there is a big push for grid connect) I sort of like the Victron units this minute. Could you suggest a few units I could investigate further. Mppt technology I believe is a feature I would enjoy also the ability to expand or connect additional units.
     Tony Sanchez, June 25, 2011

    Thank you for your comments. You are right, Grid Connect is The Big Thing at the moment, but there is still some very good stuff for off-grid systems around.

    Victron equipment has a good reputation: they are well built and people who have them tend to like them. The only negative thing I have heard about them is that some models can be a little noisy unless they are put into a cupboard or a separate room. That said, I have not experienced that myself, the Victron units I have seen have been virtually silent.

    If you are looking for alternatives, I'd recommend looking at the Morningstar equipment, which is very good indeed: solid, reliable, well built and virtually indestructible.

Hi Michael,
I found your site quite interesting, unfortunately I am ignorant to the subject as I have no technical background on such. I live in Malta, Europe where our weather is mostly sunny all year round. I s it possible to install a PV system to power 3 A/C units (for the summer months) and the normal household use of TV's, Washing Machines, Comps and normal room lighting?
What should I shop around for?
Thanks so much for your time, and patience.
     Charles Vassallo, June 26, 2011

    You can, but quite frankly running air conditioning with solar power is expensive, simply because traditional air conditioning units consume so much power.

    The average household in Malta consumes 9kWh of electricity per day. To put this into context, if you have three air conditioning units, you'll be using that per hour in your home!

    To power the average household in Malta completely on solar energy, you would need a 2-2½kWh solar array on your home, which would cost around €5,000-€7,500.

    In some European countries, there are 'feed-in' tariffs and tax breaks to help with the cost, but from what I can make out, this does not apply to Malta. However, with Malta committing to produce 10% of electricity from renewable sources by 2020 and a current capacity to produce only 0.2% of electricity from renewables, that may well change in the near future.

First, thanks for writing this book!
Here is my problem, I am a complete solar power novice and am looking at having an emergency power battery as a backup, around 400W+ (the 600W Duracell is too heavy for me) powered by a solar panel as my first time project. The "power pack" batteries generally have some sort of controlling device in them already, so do I need just a standard solar panel or one with an inverter/controller in it? Also, do you think a 50W panel is overkill for powering such a battery? Any input at all, including nixing this whole idea and steering me in another direction, is greatly appreciated. Sincerely, Heidi
     Heidi, June 27, 2011

    Thank you for your comments. You've got two choices - either build something from scratch, or use a power pack as you have already mentioned. So long as the power pack automatically cuts off charge to the battery when the battery is full, you won't need a specific charge controller for the solar panel when used with the power pack.

    The size of the solar panel required depends on how quickly you want to charge the batteries up again: the bigger the solar panel, the quicker you charge up. However, if it is too big, you'll end up 'wasting' electricity because the solar panel will be generating power even when the battery is full.

    If you completely drain your 400 watt battery (which usually means draining it to 80% of its total capacity), it will take a couple of days in the middle of summer to recharge it using a 50 watt panel. How long it will take in winter will depend on where you live: check out the solar irradiance calculator to get a better idea of how long it will take for your region.

do solar panels reduce the heat gian. I am in el paso tx. The panels should provide shade to the roof?
     craig j cummings, June 29, 2011

    Solar panels heat up quite considerably in sunlight. One of the big issues with solar design is to minimize this gain, which you do by putting a gap between the solar panels and the roof.

    Designed well, there will be a reduction in heat on the roof itself. However, it will not be a significant difference.

On your solar calculator.
1) Am I right in assuming that these are actual figures which include the average effects of clouds etc.
2) The figures for London for your “year round” tilt of 38 degrees give an average per month of 2.8975 whereas the figures for “best summer” give 3.001. Looks like best summer gives a better overall result than year round. The same result applies to NZ as well. You say that you have chosen best March /September for year round. I conclude that if I want most energy out over the year I chose “best summer” rather than “year round” tilt. Could you comment?

     martin lowson, July 5, 2011

    1. Yes, the figures take into account average weather conditions.

    2. In some parts of the world, you are right- you get more energy out of the system using a summer setting than you do on a 'year round' setting. However, that is only half the equation.

    If you want a system you can use around the year, you need to choose the right setting to generate power when you need it. For example, if you have an off-grid system and you want maximum power during the winter months, using a winter setting is always going to give you the best results, even though your system is less efficient during the summer.

    A summer setting will give you very little energy in the winter, which means that you'll have a system that is only really working for you for six months during the year. For most off-grid systems that is no use at all.

    If you have a grid-tie system, then you can earn more money from a feed-in tariff using a summer setting, but you're reducing the environmental benefit of your system in the process: demand for electricity is low during the summer months and if you are not using the power yourself, but selling it back to the grid during the days, then the value of the electricity you are actually providing, in terms of saving carbon dioxide emissions, is zero.

Plz give me a detail in formation for 500w solar power solution(calculation base).what type of batterty,inverter,charge controller,panel etc i should use.
     nizar, July 7, 2011

    Fill in the Solar Project Analysis questionnaire at The report will tell you what you need to know.

    Then take this to a couple of suppliers of solar equipment in your area and they will be able to tell you what equipment is available in your region and provide you with prices.

I am unable to face solar panels due south due to roof orientation constraints. Is there a calculator to determine solar insolation for panel orientations other than due south?
     Doug Perl, July 7, 2011

    We are currently working on having a non-south calculator on our website. It should be available in around one months time.

I am planning to fit pv panels to the South elevation of a roof. The roof is the rear section of my property with a valley and another roof of exactly the same height in front of it. Some installers say its OK others not. I need to know the height of the sun to calculate how much of the rear roof will be shaded by the one in front at different times of the year (I know now that there is no shading in Summer) . Both roofs are approx 45 degrees and approx 3m high.
Also one installer has suggested using micro inverters
what do you think?
Many thanks in anticipation of your response.
     Mel Copley, July 9, 2011

    My Solar Angle Calculator ( will allow you to identify the angle of the sun at different times of the year.

    I checked your website to see where you are based. As well as tempting me to come to Derbyshire for a holiday with the family :), this has helped me to identify your site on Google Earth. I presume you are talking about the two south-facing barns at the rear of the property?

    In nearby Derby, the angle of the sun in the depths of winter is only 14°. In the height of summer, it is 60° and in mid-March and mid-December it is 37°. Look at the solar angle calculator to get more detailed information.

    You should now be able to draw up a diagram of your roof and of neighbouring buildings to work out whether you are going to be affected by shading or not.

    Micro inverters are an interesting technology and can be quite effective for smaller installations, or in installations where shading can be an issue. The idea is to have an all-in-one unit of solar panel with inverter all in one, and if you have multiple solar panels, you have - in effect - multiple solar installations all linked together.

    I suspect that your solar installer has suggested micro-inverters for your installation because of partial shading at different times of the year. In this scenario, a micro-inverter system would probably be more efficient than a traditional configuration because an entire solar array will be affected by partial shading where as a micro-inverter system where some panels are in sunlight and others in shade will often produce more power during these periods (I say 'probably' because, without knowing your exact site, I cannot be absolutely sure).

I am based in the UK. I bought your book found it very useful. You have explained technical things in very simple way. I want to start business in UK in solar pv and solar heating. Can you please guide me that do need to know before starting this business. How is market in UK. How much I need to invest. Is there any business plans which I can go through? I look forward to hear from you soon.
     Haroon, July 9, 2012

    Thank you for your comments.

    If you want to start up your own business in this area, there is some background skills and qualifications that you will need before you get going.

    If you want to do Solar PV installations onto buildings, you will need to be a qualified electrician with sound knowledge of building regulations and 17th Edition Wiring Regulations, and you will need to have training in solar PV. Your business will then need to be MCS certified. This will allow you to install solar PV systems, connect them to the National Grid and will allow your customers to qualify for the feed-in tariffs and sell power back to the grid.

    If you want to do Solar Thermal installations, likewise, you will need to have qualifications in plumbing. Most training companies say that you should have an NVQ Level 2 in Plumbing and Heating, or two years industry experience, plus an Unvented Hot Water certificate before you apply for training.

wish to put pv on my rv.I have 17kw power requirements weekly or 2.4 kW per day average.How mnay PV panels say at 130 and 185 Wp will I require to produce Kw required and what other equipment will I need.
Much Obliged Sean From Ireland
     sean, July 10, 2012

    2.4kW a day is a lot for an RV. I'm not sure you are going to have enough roof space.

    Fill in this form with your exact requirements:

    I would suggest you try and work out in more detail what you are using your 2.4kW for and break it down, but for an initial run-through feel free to use 'RV' as the Device Description, put '100' watts in the Power column and '24' as the number of hours a day you use the system.

    You will then be e-mailed a detailed description of exactly what you will need for your RV.

We are at at 32 degrees South. Perth W Australia latitude. Please tell me what different output could be expected from a 5 kilowatt unit during summer and spring with the panels set at (from horizital) 10degrees : 20 degrees ; 30 degrees. The roof is about 10 degrees from the horizontal.We have had fitted a 5kw system to supply the house and sell the rest back to the power supplier through the grid. Would it be worth having a frame built to get the extra summer sun? Our greatest consumption of power is summer time. Thank you for your help.
     Gordon Borlase, July 15, 2012

    Your average insolation for Perth over the period of a year at 10 degrees is 5.86kWh/m2/day, with average summer insolation of 7.1kWh/m2/day.

    At 20 degrees is 5.86kWh/m2/day, with average summer insolation of 7.2kWh/m2/day.

    At 30 degrees, the annual figure is 5.87kWh/m2/day, but the average summer insolation is down to 6.9kWh/m2/day.

    The difference between a 10 degree tilt and a 20 degree tilt in the height of summer would be around 400 watts per day. Over the period of the year, there would be no real difference in total power consumption.

    The 30 degree tilt would produce considerably less power during the summer - around 1.2 kilowatts per day - but would be marginally better during the winter. However, you would only be generating an additional 12-15 kilowatts of electricity over the entire year by building a frame. It isn't worth the extra cost!

why is the online solar calculator appliances space limited. i have tried to add further appliances but i couldnot do so. Please can you help me.
     Eugene, July 16, 2012

    We are updating our solar calculator at the moment. The new version has the ability to add many more appliances.

I am in the planning phase of building a house in Lahore, Pakistan. In Pakistan there is a huge shortage of electricity and the grid is very poorly managed and in poor condition.
I just ordered your book from Amazon. What other advice would you give about perhaps wireing differently when building a new home
     Bacher Muzaffar, July 21, 2012

    Because of the high number of power cuts, you may want to consider wiring your house so that you have specific circuits that you want backed up by solar - so that if you get a power cut, you can keep essential appliances and lights powered up without wasting electricity on non-essential requirements.

    This makes it easier to implement a solar PV system to ensure you get the best use of it when you get a long term power failure.

Sir My question is :
If the irradiance is increased whteher the capital cost of solar panels would decrease or not, so plz tell me if there is any relation of irradiance with capital cost of solar panels
     hassan, July 22, 2012

    If the irradiance is increased, you need to purchase less solar panels to produce the same amount of power.

    To work out a rough figure for the amount of solar panels you need, divide your total power requirement by the irradiance figure for the weakest month of the year: that will tell you how big your solar array needs to be.

    (In reality, the solar array would need to be slightly bigger than the figure this simple calculation will give you, because there are other losses in the system, but it does give you a base figure to work from).

Hello Michael,
The inverters i see here in Nigeria seems to have a battery charging system.How do i make the connections knowing that solar would charge the batteries too from the other end?
     Dike, July 23, 2012

    I do not know of the specific inverters you refer to, so it is difficult to give an absolutely accurate answer: you would need to read the instructions for that particular item of equipment.

    However, it sounds like you are referring to a combined solar controller/inverter, where the controller puts charge into the batteries, but stops the batteries from becoming overcharged, and the inverter takes power and converts it to high voltage AC, using power from either the solar array directly, from the battery bank or a combination of both.

    There are a few of these on the market. They are usually wired so that everything is wired through the solar controller/inverter: the solar array, the battery bank and the appliances that you are running.

hi. I have a 160 watt , 24 volt solar panel and would like to know how many amp hour 12 volt batteries I would need for a panel that will be used in europe and north africa.what regulator would you recommend?
the panel is to be mounted on the roof of a van.
I look forward to hearing from you with any useful information.
Thanking you.
fran Haynes
     fran haynes, July 26, 2012

    On a good Summer's day in Southern Europe and North Africa, you'll be able to generate around 40 amp-hours of energy per day from that solar panel.

    Obviously, you won't get that all year round, but from a sizing point of view, you ought to ensure you go for at least three days energy store - i.e. a 120 amp-hour battery pack.

    However, the other side of the equation is how much energy you are using each day. Ideally, you don't want to discharge your battery pack any quicker than 1/20th of its total capacity per hour - i.e. for a 120 amp-hour battery pack, you don't want to discharge it any quicker than 6 amp-hours per hour.

    With regards to regulator, you're only looking at a relatively small and simple system. I would recommend an MPPT controller, at least 10 amps and if it were my money, I'd probably buy a Stecca Solarix MPPT-2010.

Hi Michael,I am currently designing a mini solar power system to run a few AC loads. My questions are:
(1) What is the best choice for solar battery in terms of low cost and good performance ?
(2) How does the inverter control the input current into any AC load?
(3) Which type of charge controller and inverter that is the best for mini scale systems?
Thank You Very Much.
(3) Is it
     Eason, July 27, 2012

    Batteries - depends on your exact system, but I personally like the Trojan range of batteries. They are an excellent, long lasting battery with excellent charge/discharge characteristics.

    Inverters - the inverter will include a low-voltage check, which will shut down if the batteries get too low. This stops the batteries from being damaged by discharging them too deeply.

    As to which charge controller and inverter is best, that will depend on the exact size of your system. For smaller systems, both MorningStar and Stecca make good quality kit and I can recommend both.

how are cable sizing done for solar pv to coimbiner box and from coimbiner box to inverter.there are lots of tools in net which has has confused me a lot
     AJIT S, July 27, 2012

    It depends on the length of the cable, the voltage and the current running through the cable.

    If your cable is too thin, you restrict the amount of current that goes through the cable and you get a voltage drop because of the resistance.

    If you increase the voltage of your system, you reduce the current that goes through the cable, which means you can have thinner cables, or extend the length of the cables. Every time you double the voltage, you can halve the current.

Hello Michael,
Hope you are fine, I am designing a hybrid system for the Telecom companies.
This system consists of two sources of energy. Solar panels and Electrical generator.
The output will go the batteries of course.
I am looking for one controller that could manage the current inputs from the batteries and from the solar panels.
Example: When the sun is high, the controller can reduce the output from the generator to reduce the fuel consumption and vice versa.
Awaiting your reply,
Best Regards,
Gabriel Lailo
     Gabriel Lailo, July 29, 2012

    MorningStar have a couple of high end controllers that can start up a generator automatically if the charge gets too low, as do Outback.

    Generator suppliers can also advise you on solutions for charging batteries, working in conjunction with solar power.

Can we get long term profit from solar energy? how many investment for one kilo watt solar energy in india?
     nandkumar avachat, July 29, 2012

    Depending on where you are based in India depends on what packages are available for you: there is no nationwide solar scheme, but many regions have implemented either feed-in tariffs or subsidies to help people install their own solar energy systems.

    The Indian Renewable Energy Development Agency (IREDA) does have a fund available to allow you to lease purchase a PV system. In addition, State Utilities are currently mandated to buy green energy via a Power Purchase Agreement from Solar Farms, although I believe the current agreement comes to an end in 2014.

    You would be best talking to your local government office to see what they are offering and to find out the latest information from the IREDA.

what is the minimum temperature required for a solar pannel to generate electricity
     sheikh, July 31, 2012

    As far as I am aware, there is no minimum temperature - in fact, the lower the temperature, the more efficient solar PV works as the temperature coefficient of power means the efficiency of the solar panel decreases as the temperature goes up.

    Solar PV has been successfully implemented in very low temperature environments without an issue. Satellites in space are powered by photovoltaics and often get down to temperatures below -200° celcius (-328° fahrenheit) without a problem.

I have a requirement to log the available solar raditaion strength at my location for a few days throughout the year. Can you recommend any make, model or manufacturer of this type of meter and what measurement units or range of units would I need to be sure this meter includes to get useful data?
     Billy Whiten, August 2, 2012

    I have a couple of different meters that I use for this purpose. For simple spot checks, I use a Tenmars TM206 Solar Energy meter which I simply point at the sun from the location I need to check and get a solar irradiance reading as a watts per metre reading.

    I also have a PCE-SPM 1 solar radiation meter which includes data logging. This can be installed on a site and left to record radiation over a one to two week period.

Hi Micheal,
I have just completed a course in P.V installation recently.
Now I have read somewhere and can't for the life of me remember where, but have been asked by potential customers "How efficient will my Solar Array be if it is mounted East/ west mounting"
because south facing is not possible.As a percentage I think it was around 12% less efficient
Can you verify this for me please..I know that a lot of factors will need to be taken in to account but as an approximate percentage would be close enough or details directing me to info.
Many thanks for your time
part way through your 2012 book ...very informative and crystal clear!!
     Darren Stearn, August 3, 2012

    As you say, there are various factors involved. During the summer months, the efficiency drops by around 10% but in the winter this can be as high as 25-30%. Overall, you can expect a reduction in efficiency by around 17-18%.

I am installing a stand alone system on my recently restored spanish farmhouse. I have purchased 4 x 230w solar panels. 1650 x 1000. Is there any probem mounting them so tha the 1650 is th heght and 1000 is the width? Maybe a silly question but they would fit the available roof space much better. Great book and website bythe way, i find myself constantly refering to it!
Regards Ian, Andalucia.
     Ian Burton, August 4, 2012

    Thanks for the comments. No, there is no problem mounting them either way up.

I want to run dc appliances directly from the output of panels(just when sun shines). Is it possible or do I have to use charge controller to control the output of panels. I was thinking of using a single battery in parallel to stablise the output of panels.
     Hamid Saeed, August 7, 2012

    Yes, you can run a DC appliance directly from the solar panels themselves. However, the voltage of the solar panels will vary dramatically depending on the intensity of the sunlight, and your DC appliance will need to handle this. If it cannot, you will need some form of voltage regulator to handle this for you.

    If you are using a single battery, then you ought to have a solar controller to stop that battery from overcharging. If you do not do this, you risk the battery being permanently damaged, or worse, risk fire or explosion.

We have a small solar electric system for our camp in Northern Maine, USA. Power storage is with a 12VDC deep-cycle Marine/RV style battery. We typically are only at camp on weekends during the summer months. What is the best way to leave the system while we're away? OFF? ON? ON but with a low power draw item running to cycle the battery daily? Looking forward to your response. Thanks!
Mel Clauson
     Mel Clauson, August 8, 2012

    Keep your system switched on. Having a low power draw would be of limited advantage, but the system must be kept on to ensure your battery charge is kept up during the rest of the year - especially during the winter months.

    Ideally, you also need someone to check the water levels in your batteries every four months or so and top them up when necessary.

Hi, I am trying to find the link to "Solar Insolation Values - Rest of the World" as stated on page 165 of your book. Can you please point me in the right direction on your website.
     Reiner Kuske, August 10, 2012

    You can find the information here:

which is better a solar panel with tracker or a fixed one? what is kwh/m^2/day? is it possible to have solar trackers on the roof?
     nelle, August 14, 2012

    You will get greater efficiency with a tracker, but at much greater expense. Yes, you can have a solar tracker on a roof, but the installation costs can be quite considerable.

    In almost every instance, it is cheaper to buy additional solar panels rather than to install a tracker, and therefore increase your power generation capabilities that way. Solar trackers only normally make sense where space is at a premium or site-wide shading is an issue.

    The kWh/m2/day figure is a solar irradiance calculation, showing how much energy we get from the sun over a period of a day. We can use this figure to calculate an approximate energy generation figure from a solar panel. For example, if you have a solar irradiance of 4 kWh/m2/day and a 100 watt solar panel, multiplying these two figures together will show you how much energy your solar panel would generate over a period of a day (in this case 400 watt-hours per day).

    This figure doesn't take into account other factors, such as system efficiencies, but is used as part of the overall figure to work out the size of system you require.

I'm thinking of installing solar panels, and done a bit of research into solar panels, I live in South Wales (UK), a typcal day would be Cloudy / Rainy, I believe I have narrowed my choice down to either Sharp NU245 or Sanyo 250 HIT, for a 4KW roof mounted system, @ 35 deg pitch, South East facing. Both systems will give me an appox 4kw output but for obvious reasons I need to maximize my return on my investment, with this in mind which of these two suppliers produce the most wattage in low to medium irradiance levels, and do you know what tis output is, I beleive in good irradiance conditions both systems should produce approx the same, is this also correct ?
I have the space available for both systems (approx 26Sq meters vs 21Sq Meters), and understand that the Sanyo HIT system is more expensive to the sharp system. Before taking the plunge I need to know which product is the most cost effective in terms of investment vs output, or do you have another system in mind ?
Thanks for taking the time to read and respond... Gary

     Gary, August 14, 2012

    Apologies for the delays in responding to you. I've just returned from my holidays which I spent in South Wales. I can certainly vouch for the clouds and rain!

    You've chosen some good panels there. Both Sharp and Sanyo are good makes and their systems are both very good. Whilst I have no comparative figures between those two exact panels in poorer conditions, as a general rule of thumb, the larger the surface area, the better poor-weather/poor lighting conditions performance you get. That said, the differences are usually fairly marginal, so you would probably be only seeing a difference of a few watts per day between the two different systems.

hello mate how are you i would like to hook up a solar panel to my 12v boat battery to give it a bit of a charge while im on the water what would you recomend 20w 40w i am new to this as it may be a bit cheaper then a new battery thanks waiting for your response
     mathew, August 15, 2012

    Are you just after a simple solar panel to keep your battery topped up between uses, or a bigger panel to generate all your power when you are on board?

    If you are after something just to keep your battery topped up and working well, you'll probably find a 10-15 watt panel will suffice. If you want to be able to run full electrics on board without running your engines, you'll need to take into account how much electricity you use each day. You can then use this to fill in this questionnaire: which in turn will tell you exactly what you will need to be entirely solar powered.

I live in Rhode Island and I'm curious how you calculated your values for solar irradiance? Are they modeled/interpolated from actual time series measurements at specific locations?
     Danny, August 16, 2012

    The figures are based on air and ground measurements taken by NASA satellites over a 22 year period. These measurements are taken every three hours across the surface of the earth.

There are roumers that the FIT tariff in the uk is going to be dramatically reduced next year for domestic and commercial,do you know if there is any truth to this or will it just drop 8% per year for the forseeable future
     Richard Blundell, August 24, 2012

    Yes, there are going to be significant changes to the UK Feed In Tariff next April. This will not affect people who have installed systems under the current scheme, but will affect people who want to install systems but who have not done so by next April.

could you advise me on a couple of points . What is the ideal angle for an on roof system and if 1 0r 2 of the panels are shaded does that effect all of the panels
     nick, September 1, 2012

    The ideal angle for an on-roof system for all-year-round performance is 90 degrees minus your latitude.

    In a standard series-connected system, if some of the panels are shaded, this pulls down the power efficiency of your whole array. The way to resolve this is to have multiple strings of solar panels, with each string connected to a separate feed in a solar controller or solar inverter that handles multiple solar array strings.

Hi I am living in Mudgee NSW, I am one of the unfortunates who through my installers incompetence missed getting a Solar feed in tariff. I do benefit slightly from having decided on a Net connection and not a gross connection. My 2.1 system is feeding to the grid about 5 to 6 kWh daily, with any feed-in going to the grid with NO $ benefit. Sorry, my question. How can I utilise day time power and have it usable at night. Can I use a UPS feed from a power point during the day and at night plug into it for other appliances. Are there problems with this form of storage. Any other ideas? Cheers
     Kevin, September 2, 2012

    I'm guessing that your solar array connects directly to an inverter which in turn is connected directly into your household electrics. Usually this means your system is wired in series, so it is running at quite a high voltage.

    What you would need to do is rewire your system so that your solar array charges a bank of batteries, running via a solar controller. The inverter then runs from your batteries and you remove the grid connection from the inverter so that it simply powers your home.

    Of course, there are various different configurations, so that you can retain the grid-tie if you wish, and so on, but in principle, that is what you will need to do.

I just finshed reead the 2012 edition.
Question, when sizing you wiring, do you not have to take the distance to and from you battery bank or array. I have wired some of my boat wiring and read that the distance is from the battery and back for length.
     Kerry Hansen, September 6, 2012

    The length is the physical length of the wire, not there and back.

Hi Michael, Firstly dont know anything about solar panels. From a Solar Panel.... Can i produce hot water for a shower and general use for my rural mobile home or is the outlay too much ? what would a 60 watt solar panel run.? many thanks
     andrew carlin, September 7, 2012

    OK - there are two types of 'solar panel'. There is the solar heating system, which heats water from the sun and there are solar PV panels that generate electricity.

    Solar heating systems are typically 50-75% efficient. I don't know where you live in the UK, but as a rough rule of thumb, here in England, you could expect a one metre square solar hot water system to heat up 20 litres of hot water by 30 degrees in the middle of winter, whilst in summer, you would be able to heat up 120 litres of hot water by the same amount.

    If you are looking to fit a solar hot water system to a mobile home, the outlay is far lower than it is for a house. Shop around and you should be able to find a suitable system for around £500-600.

    Solar PV is much less efficient, unfortunately. As a rough rule of thumb, a 60 watt solar panel would generate around 40 watts-hours of energy per day in the middle of winter, and around 275 watt-hours of energy per day in the height of summer.

I am trying to install a 4 camera security system with a wireless SIM card up-link for viewing the cameras remotely. I am going to need about 20 amps worth of solar to power all the equipment, I think? Question is how do I make sure to match up my watts or amps of power needed with a solar system?
     Dennis Selzer, September 10, 2012

    You can calculate your wattage requirements by multiplying your amps by the voltage you are running the system at. Then you can work out your requirements by entering it into the Solar Calculator available here:

Hello, thanks for your angle calculator. Solar panels mounted on my roof (flat/reflective) would complicate maintenance. A pilon system w/axis adjustment would be preferred. In fact, due to unusual weather, a prized tree on our front lawn died. The tree's location would seem ideal for a pilon/solar panel system, but since it is on the front, making it an art object rather than an industrial item would be appropriate. Has this been done? Since my dwelling (single story) is across maximum sun exposure (which is why I need angle data), I might need elevation control in winter. Is this available?
     Dan, September 13, 2012

    Your project sounds interesting. I haven't come across a solar installation being created as art before, but I'm sure somebody has done it somewhere!

    Elevation control is available using a solar tracker. Various models are available, but I suspect you may need something to be custom built if you want it to look artistic.

I would like to install a solar panel on my boat. I have checked out the angles for my area for best reception of the sun but as we know a boat at anchor will move with the tide, currents and wind.
my question is 'will it work but with possibly less efficiency ?'
i am going to go for the biggest panel, 120 v, that will fit on my boat...the only draw is mainly the 12v fridge, stereo and possibly a light for an hour or so. the use is mainly for the summer months as i am on the dock for 8 months.
     Bill Thomas, September 19, 2012

    The answer is yes, it will work. Mount the solar panel flat - or even better, at a slight angle so that rain water will run off - and you'll normally get around 80-90% of the efficiency of a tilted panel.

Dear Sir,
Please explain the difference between Kwh and Kwp, and how to design solar power required to operate a AC submersible pump in accordance with its HP.
     Hanumanthu, September 22, 2012

    kWp stands for 'Kilowatts Peak' and is the peak performance of a solar panel under laboratory conditions. If you see the power rating - typically shown as 100w - on a solar panel, this refers to its kWp output.

    In a live environment, the kWp output is just one of the variables that you have to work with in order to identify the true performance of a solar panel. You need to know the solar irradiance of your location - i.e. how much sunlight you get, and the intensity of that sunlight over the period of an average day, you need to know the angle of the solar panel, carry out a shade analysis and take into account various losses in the system.

    kWh stands for 'Kilowatts per Hour' is a unit of energy - power over time. i.e. A 1 kilowatt (kW) appliance consumes energy at the rate of 1 kilowatt per hour.

    With regards to your submersible pump, you really need to know its kW rating. HP is typically an output figure, i.e. how much power the pump produces, where as a kW rating is usually an input figure, i.e. how much power the pump uses. As a rough rule of thumb, 1HP equals 0.75kW of output power. At a guess, I would suggest the input power is about 20% higher.

    I would recommend that you fill in my Solar Project Calculator questionnaire (available here: This will then produce a document giving you a lot more information about how to implement your system.

I recently on Nov.24th 2012 mounted, a 3200wat solar system using 3x45amps Morning Star charge controller,
180 volt dc connected in three series method, which summed up 96v,96v and 72v panel voltages to the 3 controllers connected in parallel as it was initially, with a battery input voltage of 60volts (i.e 5x12vx100Ah) per controller, and after 3 months, the system controllers stopped given out charging voltage above 60 volts controller output unlike the initial time of the installations (that was 76volt dc), and all the batteries lowered to power the 10kva inverter system coupled to it. What might be the reasons for the failures of the controllers to give out their initial output voltage which was 76 volts charging output as it were before in the time of the installations? I have used the same method of installations in another 3 different areas that are still working perfectly up till date. thanks.
Engr. Gabriel E.J
     Engr. Gabriel. E J, September 22, 2012

    This is a very strange problem.

    How are you taking power out of the battery bank? Is the power being taken out through one or more of the charge controllers, or through a separate inverter?

    If the power is being taken through one of the charge controllers, I am wondering if there is a conflict being caused by having multiple controllers charging the batteries with different input voltages going into the controllers, and in particular with the lower panel voltage on your third string.

    Otherwise, it is just a case of isolating each part of your system and testing it individually to see what results you get. Check the input voltages from each string into the controllers and make sure they are giving you the figures you expect. If one of them is low, you may have a fault on the string which is causing the problem: then check wiring resistance and outputs from each panel until you've identified the fault.

    If that doesn't resolve the problem, I would suggest disconnecting each string in turn and seeing if the input voltages into the batteries then goes up using the other two strings. Start with the low voltage string first and then work on the other two. If you find you have one rogue string, swap the controllers around and see if the fault stays with the string, or moves with the controller.

    If the fault moves with the controller, you may have a faulty controller. If the fault stays with the string, you need to check that string again and make sure you don't have a resistance issue.

I want to know how many photovoltaic panels I will need to power a decent sized electric outboard motor for a mid sized craft; 30' to 50'. Two or more engines no problem, I just want to be as eco-friendly as possible as I run a fishing & kayaking outfitting service.
Thank you for your time,
Matthew Joyce
     Matthew Joyce, September 22, 2012

    Define a 'decent sized' electric outboard motor! I'm not a boat person, other than I've got a small electric 'Minn Kota' outboard motor for my 8' inflatable for messing about on the river. I have ideas of building my own boat one day with a small cabin and making it entirely solar powered, but right now that is still a long way down my 'to do' list!

    I've had a chat with a local boat hire outfit who rent out electric boats to find out more. Most of their boats are between 15'-25'. They've used 'Minn Kota' and 'Torqeedo' outboards in the past, but found the Minn Kota's to be underpowered and unreliable for all day regular use and the Torqeedo's to be vastly better. However, these are unlikely to be powerful enough for your needs and I've been told that the 'Aqua Watt' motors are supposed to be the best of the best.

    Unfortunately, I've not been able to find out much about these motors, other than they need a fair amount of batteries and use a lot of power. I suspect that you would probably need to use solar to supplement the recharging, rather than replacing it altogether.

Hi Mike I would to know how many 230 canadian solar panels I can put on a string with a suny boy 7000
     greg, September 23, 2012

    I'm assuming these are 230 watt solar panels? The Sunny Boy 7000 has a maximum recommended power input of 8750 watts, which equates to 38 panels.

    My personal preference is always to over-specify my contollers and inverters, so my own personal recommendation would be to put no more than 32 panels onto a Sunny Boy 7000.

My home load is 6500watt, and one of the company have a panel rating of 46watt per module. than how much panel is required to hook up a load of 6500watt?
I solve it in two methods? kindly you tell me which is right or wrong?
According to my calculation:
6500(load)*30.41(Days in a month)= 197.665kw
And the output of 46watt solar Panel (in 14 Hours of Sunlight daily) per month is= 46watt*14Hours*30.41Days= 19.58kwh per month.
Total panel Required= 197.665kw/19.58kwh= 11 Panels of 46 Watt is Required
or: 6500watt(home load)/46watt(one solar panel Rating)= 142 Panels is Required
Is this calculation right or wrong?

     Jigs, September 23, 2011

    Both calculations are wrong, but you're not too far out.

    Firstly, your solar module rating is based on the peak performance of the solar module under laboratory conditions, based on a 1kW/m2 irradiance - in other words, overhead sun at high noon in the middle of summer.

    In order to work out the actual solar irradiance in your area, you need to look this up on a table. I've got one on my site here: Select your location and the system will tell you the irradiance figures, broken down per month. You can then modify these figures by selecting which angle you plan to mount your solar modules at.

    Obviously, you get more sunlight during the summer than you do in the winter. This means that to generate as much energy in the winter as you do in the summer, you need more solar modules.

    Assuming that your energy usage is 6.5kWh per day, you can then calculate approximately how many solar modules you require by multiplying the size of your solar module by the average daily irradiance figure.

    I don't know where in the world you live, but let us assume for the moment that your irradiance figure for the height of summer is 5 kWh/m2/day and in the depth of winter, the figure is 1 kWh/m2/day.

    We can then use this to create the calculation. Incidentally, we don't need to work it out over an month, calculating it out over a day gives you the same result:

    Summer Calculation

    Load: 6500 watt-hours per day

    Generation per module: 46 watt (solar module) x 5 kWh/m2/day (irradiance) = 230 watt-hours per day

    Calculation = 6500 / 230 = 28 panels

    Winter Calculation

    Load: 6500 watt-hours per day

    Generation per module: 46 watt (solar module) x 1 kWh/m2/day (irradiance) = 46 watt-hours per day.

    Calculation = 6500 / 46 = 141 panels

    Finally, this is only a theoretical figure. It doesn't take into account any losses in the system caused by wiring, storing the energy in batteries, or inverting the power to AC household standards or so on. To get more accurate figures for that, you'll need to do a lot more analysis work. You can get a starting point by filling in the Solar Project Analysis questionnaire on this site, which will automatically generate a report on what you will need to look at:

I was searching your website and calculating our project I was wondering if it is possible to using 3 solar cell with different direction instead of one and parallel them with one battery in this way one battery in morning charge and other does not and vice versa in evening.
I am looking forward to hearing from you
Best Regards
     hamid, September 26, 2011

    You can do this, yes, but actually, you'll get better results if you face all solar panels in the same direction - i.e. south. The intensity of the sunlight is far greater in the middle of the day compared to the early morning or late afternoon.

    For the early morning and late afternoon, you'll also find that having three panels facing south will give you better performance even at these times of the day, compared to having one panel facing into the sun, one pointing south and one facing completely away from the sun.

Hi Michael,
I've just purchased your book and cant wait to ask a few questions.
1-The DECC formula for the FIT is based on BRE Sap v9.9 which is located for the exercise in Sheffield.I live in South Wales, so I can I do a realistic comparison with this pretty vague assessment.
The Govt need to give us realistic information in which to base our assessments. Is it feasable to use Met Office daylight hours to work out my regions realistic returns?
2-Also give the different panels out there. Where can I find a good comparison between companies for my region, and what returns they can give.
As you can see, I'm new to this industry but just looking for your book to arrive, and some experience in this opportunistic industry.
Many Thanks
     Chris Thomas, September 26, 2011

    Thanks for your questions. The DECC formula is pretty vague, but okay for the fairly basic calculations required for the Feed In Tarriff. The whole system is based on payback rather than environmental performance, so it is accurate enough to provide the information that is required to fill in the paperwork.

    If, on the other hand, you want to be more accurate, use the tools on this website!

    Met Office daylight hours are not the right tool for calculating realistic returns. You need to know the irradiance figures for your location, which you can find here:

    With regards to the different solar panel manufacturers, there is little but anecdotal evidence to suggest where one manufacturer scores over another when taking into account diffuse lighting, shade performance and so on. I've done some of my own personal testing on a few of the makes, but has certainly not been scientific or rigorous enough to be classed as formal testing.

    That said, I have found that most of the makes all have pros and cons that pretty much balance themselves out: one panel may appear to have better performance in diffuse lighting, another may be slightly better at higher or lower temperatures, but when they're in actual use over a period of a few months, the actual difference in power generation tends to be fairly inconsequential.

I live California and am interested in installing a grid tie pv system on the roof of my warehouse building. The incoming electrical service is a 480-volt, 3-phase, 3-wire ungrounded system. I see that a lot of inverters require a neutral connection on the ac output. How can this work with our incoming electric service? Thanks
     Danny, September 26, 2011

    You're going to require quite a specialised piece of kit to connect into a three phase system like that. You're going to require system grounding, as required by Section 250 of the National Electrical Code, ANSI/NFPA 70. I suggest you actually talk to your energy suppliers sooner rather than later: depending on the intended size of your system, they may have to do some engineering work themselves in order to accommodate you.

Dear Michael,
I read your book twice! Thank you.
I have worked in the PV industry for 2 years and always felt slightly uncomfortable knowing that many were simply buying solar panels in order to make an investment. Even so, something had to be done.
However, many Solar companies disagree with your assertion
"...some people who have grid-tied solar power actually make no difference to the carbon footprint of thier home." In your book you do explain why this is.
My question (which know one seems to want to answer) is: when 'clean energy' is generated via solar panels and sold to grid, surely this energy then has to be stored via an 'accumulator' so:
1. Is this clean energy from the pv panels still clean when sold to others?
2. Is more 'unclean' energy been used in order to store the energy given from pv panels?
3. Does this clean energy sold to grid simply reduce the need for the power companies to produce their own i.e. it therefore does not matter if someone uses the clean energy they produce?
Any response is appreciated.
     Desmond McLean, September 28, 2011

    Before answering this question in detail, I must stress that this answer is very much based on the electricity demand and supply within the United Kingdom. Some of the information is relevent for other parts of Europe, Canada and the northern half of the USA, but the true environmental benefit of solar PV does vary from region to region, depending on its mix of power stations and its climate.

    Here in the UK, it is fair to say that almost every solar company will disagree with my assertion about the environmental impact of their installations! Privately, many of them have acknowledged that I am right and none of them have ever been able to contradict me. The evidence in Germany, where huge amounts of PV has been installed over the past five years, shows that there is negligable difference in the carbon footprint of the nations electricity generation.

    Of course, that would all change if we were storing surplus energy in an accumulator of some sort. Then, electricity demand at peak periods could be generated during quieter periods and stored until it was actually required.

    That happens to a very small extent at the moment, but not nearly enough. Energy is being stored in water (pumped storage). In simplistic terms, water is pumped to a lake at a top of a hill when demand for electricity is low. The energy is then released, quite literally at the twist of a tap, by releasing the water through a hydro-electric power generator when electricity demand is high.

    At present, these pumped storage systems are generally powered from hydro-electric power plants, or from nearby coal, gas or nuclear power stations. It's a useful way to store up energy overnight to release during the day.

    This is a fairly efficient way of storing energy: I'm told it is around 65-70% efficient, which for large scale energy storage is pretty good. Right now, only a very tiny amount of our surplus energy is being stored this way, however. Huge amounts of effort and money is being spent on precisely this area. If we can crack it on a much, much wider scale, suddenly solar - and wind power for that matter - can become a significant technology for reducing carbon emissions.

    Anyhow, back to your questions:

    (1) Is this clean energy from the PV panels still clean when sold to others?

    This depends on your perspective: if you own a home with a solar array, the electricity being generated from the roof of a home is still clean when it is fed into your neighbours home and used to power his appliances. However, the electricity that you take out of the grid in the evening to switch on lights, watch the television and boil the kettle is the dirty electricity being generated from conventional power stations.

    From the perspective of a power station, if demand for electricity goes down because the sun is shining, they can turn down the power generation. However, they need to be able to respond immediately if a cloud blows across the sky and blocks the sunshine for a few minutes. Which means the fuel is still being burnt in the power station, only the output from the power station has changed.

    (2) Is more 'unclean' energy being used in order to store the energy given from PV panels?

    We don't store energy specifically from PV panels at the moment, and most of the pumped storage energy capture we have in the United Kingdom is used for capturing energy from coal, gas and nuclear fired power stations at night. As an individual, we can store energy in batteries, if we so wish, which do have a carbon impact, but are still significantly cleaner than using electricity generated from the standard UK energy mix.

    (3) Does this clean energy sold to the grid simply reduce the need for the power companies to produce their own.

    No it doesn't. The power companies need to be able to cater for peak demand for electricity, which means the capacity needs to be in place and available even if the sun isn't shining and the wind isn't blowing. All power stations have an optimum performance level: run them at peak capacity and the efficiency drops. Run them on tick-over and the efficiency drops.

    If ten thousand homes in the same area all contributed solar-generated power all at the same time on the same day, the demand for electricity from the power station would drop significantly. If clouds then blocked out the sun for fifteen minutes, the power station would suddenly have a big demand for power all at the same time. Power stations cannot react as quickly as the clouds blowing across the sky, so what is actually happening is that the fuel is being burnt at the power station all the time, but the generators are being turned up and down to compensate for the rise and drop in demand, as well as supply.

    Of course, I paint a bleak picture here, and I ought not to, because actually there are some real benefits of using solar. However, it is too simplistic to say that just putting solar panels on the roof of the house is going to make a difference to the environment: sadly, it is not that simple.

    If you want to install solar in order to reduce your environmental footprint, you need to have a specific application for it. If you've got an electric car or an electric bike, you can charge that up with solar power, so long as you can park it and have it charging up during the day. If you want to install electrical power into a remote building, you can have an off-grid installation. If you have a normal home, you can build a semi off-grid system to power one or more of your circuits in your home (lighting is often a good one to start with). All these things can make a difference.

    Hopefully, one day, just putting a grid-tied solar array onto the roof of your house will also make a difference. But until we have widespread energy storage in order to smooth out the peaks and troughs in electricity demand and use green energy generation effectively, saving the planet is just not going to be that simple.

Dear Micheal
I appreciate all your helps and it was nice of you designing this site and helping us I have some difficulties with my controller, when the voltage of cell become less than 12 it can not charge my battery (12volt 7Ah)I was wondering if there is other charger (controller) which can charge even when the cell voltage is lower than 12volt
Yours Sincerely

     hamid, September 29, 2011

    Sorry - when the solar panel voltage drops below around 13 volts, it will not charge up your battery.

    The output from your solar panel should be significantly higher than that, however. I would have expected the output to be in the region of 14-16 volts for most of the day, and higher than that on an open circuit (18-22 volts on an open circuit is quite common).

    Of course, once the sun goes down, the voltage will drop quite quickly and the voltage can quite quickly drop to under 12 volts. Most solar panels will still generate some electricity through the night - the panels on the side of my house often still generate one or two volts even on dark nights - and I have experimented with charging AA batteries with them (no, don't do it, it's not worth it!).

    If your panel is generating less than 12 volts during the day, check the voltage at the panel itself to see if the voltage is significantly higher at that point. It isn't uncommon for resistance to build up in the wiring between the solar panel and your controller. If the panel is low on voltage, then unfortunately, it is that which is at fault and you are going to need to replace it.

H Michael.
We are looking into buying a 120w camping solar panel. The maximum outage is rated at 18v for this particular panel. Will it work to charge a 12v battery without damaging it? For camping purposes will a 120w panel be sufficient? What would you recommend? We have a camping trailor and the only thing we pretty much run off the battery is our fridge and maybe some lights.
Thanks for your help.
     Nikki Wright, October 3, 2011

    You'll need to put a solar controller or battery regulator between the battery and the solar panel. This will stop the battery from being overcharged by the solar panel.

    As to the size of the panel and battery, that does depend on the amount of power required by your fridge. However, as a rough guess I would say 120 watts is more than enough for most camping applications.

Hi Michael
Series or parallel connections? i am proposing to install a few PV panels on my roof (DIY home installation), i read that to connect, i can connect the panels either in series or in parallel. How would i decide which is best or suitable? I can see the benifits to each, but is there a preffered industry method?
Regards Stewart
     stewart nelson, October 3, 2011

    The preferred industry method is to connect them in series, because the greater the voltage, the higher the efficiency and the fewer problems you have with potential resistance issues.

    However, it all depends on what you want to achieve. Are you going to be using the system in conjunction with the grid, or as a stand-alone application? Are you incorporating batteries with the system?

    If you're using batteries, my personal preference is to keep to low voltages: typically between 12 volts and 48 volts, depending on the current. If you're going pure grid tie, in series can be more efficient - especially if you are looking at quite a high current.

    You also need to look at shading. If your site has some shading issues, it can be better to run multiple strings of panels or run everything in parallel, connecting each panel or string into to a solar controller which can handle multiple inputs. That way if one or more panels are shaded at any time, it doesn't bring down the power generation of the whole array at the same time.

On your web site it says "once your solar system is connected to the grid the amount you receive will be fixed for 25 years". What is fixed, the amount you are paid per unit or the total amount you can be paid?
Incidentally excellent web site, veryusefull.
     Paul Cundy, October 3, 2011

    The amount you are paid per unit is fixed, rather than a ceiling on earnings.

    There is some question about what happens if you upgrade your system during that 25 year period - i.e. whether you can earn more by adding more panels later on. I think that question has yet to be resolved.

Hi good day, I was wondering about the computation of the solar irradiance. is the computation included in the book? please reply. thank you.
     Fowell, October 4, 2011

    It isn't a computation. It's based on real data, taken from readings via satellite.

Great book- about the only book I bought from Kindle as i needed it fast. I had to smile at your use of cardboard panels. See my product Visual Building Solar Designer for alternative solution to cardboard:
This is intended for installers, but home users can still visualise solar panels on their roofs using the entry level version of Visual Building (but without the Solar calculation feature), but anyone can download the fully function evaluation versions. Maybe you should include a review of such software in your next 2012 edition.

     Les Player, October 4, 2011

    That looks like a nice piece of software. I will certainly mention that in the next edition of the book.

    Making cardboard cut-outs of solar panels is a great way to help visualise how an installation can work. Sometimes it is that visual link that can really help people identify the best location for panels.

    Looking at your website, it looks like your software allows people to do pretty much the same thing - i.e. visualise the whole installation first - but of couse then also allows them to estimate the efficiency of the system and produce a full solar installation report. Very good.

hi i have a 12v 100w and a 12v 50w solar panel can i put in series to make 24v to charge my 24v battery bank or will it now work were they are different wattages
     simon, October 4, 2011

    You shouldn't mix panels of different wattages when connecting them in series. It will work, but the efficiency will be significantly compromised.

I have 2 20+ year old solar panels that do not have a diode built into them. Is a diode necessary and will power feed back into the panels at night.
Thanks, Rich
     Rich, October 5, 2011

    Yes, a diode is necessary to stop energy being drained by the solar array at night.

Hi Nick,
I need to charge a 12v 100 ah lead acid battery. Do you think 12v 200watts solar panel can charge it fully in one day of sun light. How much time do you think it will take to charge this battery? I guess approx. 6 hrs but i am not sure. I am getting a 200w 12v solar panel but its not flexible. Will I be able to get a 200w 12v flexible solar panel. Can i also know if this type of flexible solar panel can have a customized shape, or does it has a standard shape? Can it be an organic shape also or will it be only a rectangular shape?
I need to manufacture my product which has recently won a sustainable design awards by victorinox. Here is a link to my project:
Thanks a lot!
     Anurag Sarda, October 6, 2011

    That is a very interesting looking product. You will need to talk directly to a solar panel manufacturer to get a panel built specifically for you, in order to get the shape and size that you need.

    I would recommend a company called Q SOLAR ( Not only do they have some very interesting products, they are also extremely nice people to work with.

    With regards to the capacity of the solar panel, that depends on a lot of factors, including solar irradiance levels where you are installing the unit and temperature. I would recommend you talk to Qsolar about your product first because they will be able to give you the level of information you require based around your specific product.

What %loss of irradiance should I allow for my pv installation on a west facing 40 degree roof UK N52:21:45?
My impression from your book preview is that UK locations don't offer much user gain from what is generated. True for many but for lots more, whether like me - retired - or otherwise at home during day (housewives, kids, elderly etc, etc) quite a large number can make good use of what's generated. By observation of weather, appliance loads and adjustments to habits, much benefit is possible. Apart from fuel bill economies, what is exported to grid reduces load for power supplier. I now use microwave to heat water for hot drinks, for wash & shave, plus some cooking. Waiting until pv output reaches certain levels (I have bluetooth display)I can defer some jobs like washing machine, tumble dry, ironing, hoovering etc. I did all of these today 6th October using just 100 watts from grid and exporting 10.4 Kw to it (sunny about 75% of day). I intend to use electric fire in cold periods when pv supply adequate, so saving on oil OFCH. It will all need my closer attention to and application of my energy use but potential is big. My analysis for ROI is 7 to 8 years then full benefits for life of installation. My 4Kw install cost only £10,500.
See also
I hope these examples are of interest. I still see blogs knocking use of pv technology as a waste of money. Couldn't be further from the truth for me.
Best wishes
     Bill Munns, October 6, 2011


    It is so heartening and inspiring to read stories like yours. You are using solar energy the right way - you're using the energy that you generate yourself, and minimising the amount of energy you take from the grid. Very well done.

    The issue with solar energy in many households is that many people put it on the roof of their house without thinking everything through. As part of the bigger picture, it is important to see what happens to the electricity that is exported, and to understand the carbon impact in the evening when you take energy out of the grid again.

    By minimising your use of electricity from the grid, you have obviously worked that out for yourself and you're doing something very positive about it. Very well done. Do you mind if I use your example in the next edition of my book?

    Finally, to answer your question: you should estimate an average loss of around 15% based on the angle and direction of your roof.

Why have you missed out the Caribbean Islands state of ST KITTS & NEVIS. They are in the top ten of Eastern Caribbean Tourism, and the head quarters of CARICOM and central bank of Eastern currency are located there.
I would like some data listed in the online calculator about this state.
Can add ST KITTS & NEVIS to the list of countries, so i can colect online solar solution.
ps, if you are not responsible for adding names to the list, can you bring it to the attention of those who can.
     Don Matthew, October 8, 2011

    Dear Don

    Thank you for bringing this to my attention. This will be rectified in the next two hours.

I have 4 200 watt 18 volt solar panels. They are rated at 11.5 amps. I want to charge 12 volt 125 amp hour batteries. I live in South Florida. What is the maximum number of batteries that I can charge efficiently if I drain them to 50% every day?
Thanks in advance
     Mike, October 10, 2011

    If you angle your solar panels at around 47 degrees and face them due south, you should get good performance both during the summer and winter.

    You could expect to generate between 3.5kWh and 4.5kWh of energy from your four solar panels all year round. That equates to between four and five batteries, discharging to 50% capacity every day.

can I power 1500 watts GE Dual Burner Hot Plate with a portable solar panel?
If yes what is the best portable panel I can use?

     josue, October 10, 2011

    Not if you want to do this on a regular basis, or for a long period of time.

    A 1.5kW hot-plate will use 1.5kWh of energy per hour. That means if you want to use it for twenty minutes, you're going to need a 100Ah 12 volt battery as an absolute minimum. You'll then need quite a big solar array to recoup this power - how big depends on how quickly you need to recharge the battery again and where you live.

    If you live somewhere sunny and hot, like Florida, you'd need a 150 watt solar panel to recharge your battery in one day. If you live somewhere cold and wet, like London, you'd need a 500 watt solar array in order to do the same.

    Either way, those size panels aren't portable. Sorry!

Dear sir!
using the calculators i can get the monthly average daily irradiance for my city , also optimum tilt angles corresponding irradiation, but i would like to have the formula to calculate the irradiation and the optimum tilt angle myself, also the formula that relates the average horizontal irradiation to tilted an optimum angle irradiance.
thank you
best regards
     Osman, October 13, 2011

    The irradiation is not a formula, it's based on satellite data readings, taking into account climatic conditions and weather patterns over the past two decades.

I have a 10w panel and a 100Ah solar battery,do I need a regulator or I can connect it directly from thepanel
     Aaron Kapachiika, October 13, 2011

    The panel is not powerful enough to damage the battery from overcharging, so you can connect it directly to the battery without a regulator.

    Do make sure your panel has a diode in it to stop energy leaking out of the battery at night.

well i was asked to do a project at school.the project is about designing an alternative supply(solar) to run the lighting system of a normal AC household, rated at 1KW.the design should include a battery system and a converter. i just want to know how to go about on doing it?
     walter, October 14, 2011

    Visit this page:

    Enter the information required, using the lighting as a device description, setting the voltage to 'mains voltage', setting the power to 1000 and the time to one hour.

    This will e-mail you a report that will explain exactly how you go about putting in this system.

I was delighted to come across your book, Thank you for a being so generous with your time in answering people's questions, I have a few queries. I am hoping to install PV panels, 2.5KWp and claim the feed in tariff; it is disappointing that it is not as beneficial environmentally as I had hoped. I live in Woking Surrey, however I think solar thermal and PV are my only practical options for now,
Is installing batteries and claiming the lower FIT much better for the environment and as cost effective as the straight grid connection.
Question 2, my roof is partly shaded by another part of my roof, by 8 am, half the roof is out of the shade and before 9am all the roof is out of shade, is there a way of connecting the panels to reduce the impact of shade on the overall productivity of the panels, 20% loss of efficiency seems a lot. At a recent Eco fair, I was told that the Sanyo panel has a much higher efficiency and is much less affected by shade, as the cells function individually, is this correct, and is it generally worth the extra cost. I don't have a lot of roof space only 24m squared but it is a triangular shape, as the roof is hipped.
My home is about 30 degrees off due south towards east, how much will this affect the efficiency of them?
I am assuming the panels placed at a 50 degree angle to vertical are only effective for 12 hours of the day (180 degrees) until the sun goes behind the house, and that there is only minimal sun gain after that is this correct?.
I hope you have the time to answer mu queries.

     Christine Murphy, March 23, 2012

    Thank you for your comments. Solar energy can be beneficial, but you need to plan that in: unfortunately, just sticking a solar panel on your roof, pointing to it and say - "Look! Green!" doesn't actually work!

    If you work from home, or use most of the electricity yourself, rather than just selling it back to the electricity providers, there is some justification in saying that you are being environmentally friendly. Likewise, if you live close to a city centre or an industrial park where there is high demand for electricity during the day, you can justifiably say what you are doing is environmentally beneficial, even if you are selling back most of your own energy back to the power companies.

    If you want to store some of your energy for your own use in the evenings, so much the better. Having some battery storage means two things: firstly, you've got power backup in the event of a power cut, and secondly, you're using your own energy during the early evening peak, which is when the electricity companies are at their most stretched.

    With regards to shade, you can reduce the impact on shade by having panels in separate 'strings', connected to an inverter or controller that can manage multiple strings of solar panels. This means that if one string is in the shade, you still get maximum power from the other string.

    I don't have experience with the latest Sanyo panels, but I am told by people who use them that they are very good. I suspect they work by having multiple strings inside each panel in order to reduce the impact of shading. If it works, it is worth spending a little extra, but if the cost is significantly more, you may as well buy extra panels from another supplier instead.

    Your shading does not seem too much of an issue as most of it is early in the morning. In the height of summer, you're probably going to lose 10-15% of your power generation as a rough estimate, but during the rest of the year, the shading will have minimal impact.

    I do not have the figures for a 30% skew from South, but I am working on obtaining this information and plan to have it on the website in the next month.

my question to you is how the solar angle calculator works??
on which mathematical relation it decides the best direction for the soalr system?

     Ridwan, March 24, 2012

    It is based on the longitude of your location and then calculating the height of the sun at different times of the year, based from this figure.

    The basic calculation is 90 - longitude = height of the sun at midday on the solar equinox.

    The 'solar equinox' are the two days of the year when the day is exactly 12 hours from sun-up to sun-down - 21st March and 21st September.

    The height of the sun at midday rises by 7.8 degrees per full month from winter to summer, and drops by 7.8 degrees per full month from summer to winter.

    This is how the calculation is made.

Can I use a 80 watt and 160 watt solar panels together to charge my 12 volt 35ah battery?? I have a eighty watt panel and found a good deal on a 160 watt. I would like to join them for a quicker charge if possible. Or should I get another eighty watt panel?
Thanks in advance.
     Gary Enns, March 31, 2012

    Yes, you can use the different panels together. If you wire them up in parallel, you will get a slight drop in performance on the 160 watt panel, but not a lot.

Hello Michael!
I recieved your book today, but haven't read it yeat :-)
You made some good calculators on the website, but is it possible to calculate the number of kWh per m2 per day if the panels are facing south west or south east? I know that south is best, but it is not possible ...
     Peter, March 31, 2012

    I am working on getting this information and it will be on the website in around one months time.

Why do solar panels have to be wired in stings
     Mark new land, April 3, 2012

    You wire solar panels together in 'strings' (i.e. in series) to increase the voltage of the solar array. Higher voltages mean greater efficiency, especially if you are using an inverter to directly convert the DC power from the solar array to AC voltage.

    You do not have to connect solar panels together in series. If you have multiple panels, you can also choose to connect them together in parallel, thereby increasing the current without increasing the voltage. This has some benefits in off-grid systems where you are typically charging a bank of batteries.

Your solar calculators what assumptions do they make? As in do you take into account weather patterns or just latitude and longitude.
     Arnie, April 4, 2012

    They take into account average weather patterns for that region, as measured over a twenty year period.

    I have tested these figures using live installations measured over a period of a year and the figures do tend to be fairly accurate. On a day by day basis there can be fluctuations of up to 20%, but when measured over several days, the figures do tend to average out quite well.

I'm unable to send solar calculator. Error page, the likes of which I've never seen before ...Alogorithms cluster farms etc.
Bought your book, think I struck gold.
     steven ludlow, April 6, 2012

    Thank you for your comments about the book. We have had one or two problems with our web servers recently, but these should now be resolved.

I am building a new house which has been aready wired with awg 14 and awg 12 wire which is standard for 110/120 in Canada .I am not on the grid yet but have been using a 12kw diesel plant for building and cordless equipment for building .My question is it worth while to use solar where my installationt cost for grid power is aproxx. 10-12 thousand CAD .Thanks Ken Lawrence
     Ken Lawrence, April 8, 2012

    I built my own home a few years ago, so I know how expensive it can be to get a connection to the grid - even if your home is only a couple of dozen feet away from a suitable supply.

    Certainly if it is going to cost you $10-12,000 to get grid power you should consider solar and running off-grid. However, you need to think carefully about how you use electricity. When you 'make your own', you only have a finite supply: when you've used it up, you can't just pop over to a neighbor and ask to borrow some of theirs!

    If you have electrical heating, electric cooking, lots of computers, several large screen TVs and air conditioning, running solar is not really an option. If your requirements are more modest, then solar could be more worthwhile. Use the solar project analysis tool on this website to see what figures that gives you to see how it stacks up against a grid connection.

Hello Michael,
Your book help me a lot to better understand better solar installation. I seem to have a small problem with my current installation. I have bought 3 x 100 watts solar pannels, a 3500 WATTS CONVERTER, a TriStar Morningstar charge controller and 6 x 12 volts deep cycle batteries @280min/20amps capacity. I have charged up the battery bank, connect the controller and converter as per the manufacturer specifications. For my first test I have plug my slow combustion stove fan (300 watts) to my converter. The charge controller at the beginning was indicating with its LED panel lights, that the battery were charged between 80% to 95%. After about 1.5 hr of use, the charge controller was indicating a battery charge between 0% to 35%. Quite a drop for only a short period of usage.
The only thing I have not hook up yet is the ground cable from the battery bank to the earth rod. The controller and the converter are currently grounded to the earth rod. Could that have an effect in the fast discharge? Another info is that I bought the batteries on 29 june 2010 but only installed them in late october 2010. I want to have your opinion before I do an inquiry to my battery sale person. One last thing, is there a good battery tester that you would recommend? Does a car 12 volt battery tester any good for this?
     Raymond Touchette, November 22, 2010

    This doesn't sound like an earthing problem. It sounds like you have one or more faulty batteries in your pack. A faulty battery often does not show itself when fully charged, but when placed under a load you will find the voltage will suddenly drop and the pack will simply stop providing a useful charge.

    To test your system, measure the voltage of each individual pack when the system is fully charged. Make a note of each voltage for each battery. Then run your test again, measuring the voltage of each individual battery every half an hour whilst the system is in use.

    You will probably find one or two of the batteries suddenly die at some point during your test, providing a voltage of 2 or 4 volts rather than 6 volts. It is not uncommon for a battery to actually go negative if it is very bad.

    In all probability, you are going to need to replace this one rogue battery. You can sometimes rejuvenate the battery by taking it out of the pack, charging it up individually and then putting it back into the pack. This can sometimes help extend the life of the battery, but you would probably find the problem battery would fail again after a couple of months.

I am considering installing a 4KW PV panel system. This will be about 9 meters long and 3 meters high. It would be mounted on the ground not a roof.
It will be difficult to keep the full PV panel length out of shade all day. I have been told that if any one panel is in the shade this affects and lowers the voltage on the full set of panels.
If I split the panels into three sets, shade could be avoided. What happens to the inverter in this instance. Would more than one inverter be required.
I intend to vary the angle of the panels to suit the season of the year. In Enland how much improvement approx would be achieved?
Comments would be appreciated.
     David Peacock, November 23, 2010

    You can split your solar panels into three separate arrays and connect these in parallel with your inverter. You would not require more than one inverter in this instance.

    With regards to adjusting the angle of your solar array, you can generate significantly more energy by adjusting the angles of your array throughout the year. This is most noticeable in winter when in some circumstances you can double the performance of your solar array.

    As of around half an hour ago, we now have a brand new set of solar calculators on the website that allows you to select your location and then see the difference in power generation potential based on the angle of your solar array.

    Visit in order to see this for yourself.

Hello Micheal,

I forgot to mention that I have two converters hook up to my battery bank.
One is a 110 volt and the other one is for 220 volts. Do they pull any current even if they are not turned on?

Raymond Touchette

     Raymond Touchette, November 24, 2010

    Most solar inverters do not draw any current when they are switched off. However, the only way to be absolutely sure is to measure the current draw using a multimeter.

    Remember that batteries will slowly discharge if left on their own, and faulty batteries can speed up this discharge, so if you are seeing a drop in battery capacity, that is most likely to be the reason.

Dear Mr. Boxwell,
I am currently working on a project to assess the feasibility of solar energy and am currently considering using a tracking system, one of your online calculators provides the optimum tilt angle for each month dependant on location I was wondering what equations are you using to calculate these values.
Kind regards
     Louise, November 26, 2010

    Calculating the optimum tilt is relatively easy. Find out the latitude of your position (I guess from your e-mail address that you are based in Guildford, where the latitude is 51.2).

    On two days of the year, the height of the sun at solar noon is exactly 90°-latitude. These two days are the equinox - 21st March and 21st September.

    This angle increases during the summer, at the rate of 7.8° per month, until the 21st June (summer solstice), after which the angle starts to decrease by 7.8° per month.

    So on the 21st June, the height of the sun at solar noon is 90°-latitude+23.5° whilst on the 21st December, the height of the sun at solar noon is 90°-latitude-23.5°.

Does it pay to calculate precise sun hours and solar noon for a specific location down to the min and seconds? I can't feature using averages all the way around will give best performance even if it's a fixed panel.
     Larry A W, November 30, 2010

    You do need to be reasonably accurate, but a couple of degrees out either way really does not make that much difference.

    We have just updated the solar calculators on our site, so you can actually play with the insolation figures to see what differences you get at different angles. You can find the calculators here:

what is the difference between w/m2 and kwh/m2/day
what is the relation between
     hamidi, December 5, 2010

    w/m2 is a measurement of power whereas kWh/m2/day is a measurement of energy.

    So w/m2 is the amount of power you are getting at any given moment, whilst kWh/m2/day measures how much energy you have over a period of a day.

    So a solar array with a capacity for generating 1kW of power has the potential to create 1kWh of energy per hour.

    Hope that helps.

In an off-grid Solar PV system, the batteries are fully charged, all lights and appliances in the structure are powered off, and it's high noon; Where does the energy produced by the solar cells go? Does the controller open a circuit? Are solar panel lifetimes degraded if left "charged" as in this scenario?
     Thomas, December 11, 2010

    In this scenario, the solar controller cuts out the solar panels, leaving an open circuit. The voltage of the open circuit increases and the solar panels themselves heat up: in effect, the power generated by the solar array is lost in heat.

    Some of the older, cheap amorphous panels have been known to degrade over time as a result of this, but in general this is not a problem.

Hi Michael,
Love your book. I'm located in the USA and I'm designing a un-interuptable power supply system (with and without solar) for grid loss in a remote location, where power typically goes out for 6 hours a day and I need to provide 300-600 W of continuous power. Funny thing is I talked today with MK Battery, a major gel cell manufacturer here, and they had no clue about the Charge Cycle Efficiency factor you describe. Do you know if there is a comparable but different term that is used in the USA? When I described what the term meant the technical support fellow still had no clue. You mention a CCE of approx 95% for a quality "traction" battery. Do you know what it would be for a good quality 12 V gel cell of approx 100 AHr? Thank you very much, Peace
     Jamey Gerlaugh, December 13, 2010

    Thanks for the comment about the book.

    I know MK Battery and I've used their products myself. Their gel batteries are as good as traction batteries were just three or four years ago.

    As far as I know, the term Charge Cycle Efficiency actually originated from America. US battery manufacturer Trojan use it in their technical documentation and I'm fairly certain I've seen it in relation to US Battery as well.

    I would err on the side of caution and assume a 90% charge cycle efficiency for gel batteries. In reality, you'll probably get nearer 95% because they really have improved so much over the past few years.

Dear sir
now my friend are writing thesis about solar cell istalation, and I would like to know what best angle for solar cell instalation in LAOS PDR for each month, and tell mee about calculation angle.
Touy, December 17, 2010

    Thank you for your question, Touy. Lao is included in the online solar calculators on this website and you can look up the insolation figures for the 11 largest cities in your country.

    Visit and select 'Lao, People's Democratic Republic' and then select the city closest to you from the list. This will then show you the insolation figures for this region for each month of the year.

    You can then compare the insolation figures based on the angle of your solar panels - i.e. fitting the solar panels upright, or angled towards the sun at different angles.

can you please explain the meaning of "efficiency levels" for amorphous,polycrystalline and moncrystalline panels. I don't seem to understand the percentages.
     Angel Vazquez, December 22, 2010

    The efficiency percentages show how efficiently the solar panel converts sunlight to electricity.

    On a clear day when the sun is directly overhead, the energy levels we receive from the sun are approximately 1,000 watts for every square metre on the ground. The efficiency percentages show how much of that 1,000 watts of power is converted into electricity: so, for instance, a metre square solar panel with a 10% efficiency would convert 10% of 1,000 watts = 100 watts of power.

    Of course, we don't buy solar panels by the square meter, we buy them by the power output. So what the efficiency levels show you is the approximate size of the solar panel you end up with.

    So if you have the choice of an amorphous solar panel with 6% efficiency and a monocrystalline solar panel with an 18% efficiency, then the monocrystalline panel will be one third the size of the amorphous solar panel.

Not a question, but a big thank you
My daughter gave me your book yesterday for Christmas and I just finished reading it.
I have been looking into solar power for about a month now and everything I have read so far only added to my confusion.
Your book made everything so clear I can't wait to get started.
Thanks again,
Don Sebring
Casa Grande, Arizona, USA
     Don Sebring, December 26, 2010

    Dear Don

    Thank you for the compliments. Solar power is no great mystery, but I wrote the book because getting to grips with the basics can take some time.

    Good luck with your project, and don't forget there are some useful tools on the web site to help keep things simple.

    All the best

am new to solar power concepts...what will be the size of the panel for charging a 12V dc battery.
     deepak shankar, December 27, 2010

    It depends on the capacity of the battery (which is usually shown on the batteries in amp-hours) and the intensity of the sun at your location.

    Lets assume you have 12v deep-cycle battery with a 100Ah capacity, which would be a typical battery used for many smaller solar applications. You would typically not want to discharge this battery below 50% of its capacity, so lets assume that you would only be using 50Ah of the energy stored in the battery.

    A 12v, 60 watt solar panel would be able to charge this battery up at a maximum rate of around 5 amps per hour (60 watts divided by 12 volts = 5 amp-hours). This means that around 10 hours of midday sunshine would charge this battery from 50% charge to 100% charge.

    You could, of course, choose to have a bigger solar panel to charge the battery up faster, or you could have a smaller solar panel to take a longer time.

    To work out how long it takes for you to get the equivalent of 10 hours of constant sunshine in your location, you need to look at the online calculators on this web site. Look at the solar irradiance tables for your location to see what the figures look like in your area at different times of the year. In the height of summer, you may be able to charge your battery up in less than two days. In the depths of winter, it may take a whole week for you to charge up your battery. It all depends on the climate in your part of the world in different seasons of the year.

Have you considered making your book available in PDF format? I see there is a Kindle version, mind selling me a copy of a PDF or other?
     Derek Jones, December 27, 2010

    Hi Derek

    There will be an EPUB and PDF version of the book available in about four weeks time.



hi Mike,
My brother has a copy of your book. In it, you mention that a 'solar energy saving fridge' uses 5W/hr compared to a camping fridge which uses 110W/hr. I was planning to replace my 240v fridge with a 12v camping fridge and dedicated pv panel, but am now more interested in the energy saver fridge. I have never seen such a thing for sale. Can you suggest where I might buy one (in the UK or SA), and perhaps supply a brand name or two.
Steve Jones
South Africa
     Steve Jones, January 4, 2011

    There are three manufacturers, Waeco, Sundanzer and Shoreline. Of those, Sundanzer and Waeco both have a sales and service outlet in South Africa and are probably worth talking to first - Sundanzer can be found at whilst Waeco can be found at

Are there any solar shingles available that one can use to do the entire roof instead of using solar panels?
     Iqbal, January 7, 2011

    There are, but I do not have any experience of them myself.

    A company called Dow Chemical have a range of shingles sold under the 'POWERHOUSE' brand. The Dow web site is at, but unfortunately it is one of those really awful ones that take for ever to find what you need.

    Another company who build a solar shingle is Solar Century who have a product called the C21 Solar Tile. You can find Solar Century at

    There is another product called SolarTile from a company called SolarTile. You can find them at

    Hope that helps.

hi i was wondering if you could tell me if you can use both seriel and parallel conections in a diy panellel or any suggestions thanks jim
     jim richards, January 7, 2011

    Yes you can. You can use the panels in series to increase both the voltage and the capacity, and then connect multiple series of panels together in parallel to increase the capacity without increasing the voltage.

    There are several examples and diagrams of exactly this configuration in the book.

Hello Michael,
On page 90, what is the size of the DC Circuit Breaker should I use? My solar panels would be total 285 watts! I am doing a stand alone solar project and I have rewired my house with 150 feet of 14 gauge AC wire. I plan to use a 2000 watts inverter. I am glad that I read your book first before my project. Otherwise, I will make some mistakes. Thank you very much for those great information!
     Roger Yung, January 9, 2011

    Your project sounds very exciting. Circuit breakers are rated by amps - so divide the total wattage by the voltage to get the ampage, and get a circuit breaker that is at least this size.

    Do a search on Google for 'DC Isolator Switch' and you should find plenty of products that will do the job. They tend to range in price from $30 to $150 for pretty much the same device, so it does pay to shop around.

    You should probably consider getting a ground fault interrupter as well, unless your inverter already includes one.

    All the best

Dear Sirs,
I have a fast moving stream in my garden and I intend to put a water wheel in to generate electricity.
Can you supply me or point me in the right direction for a generator and distribution box please?
Many thanks in advance.
Yours faithfully,
Simon Howell.

     Simon Howell, January 13, 2011

    That sounds like a fun project. Hydro does not work in many places because you need quite a significant head (i.e. fall) and a pretty powerful flow for even the smallest hydro-electric system.

    I'm not a hydro expert but I do know of a couple of companies who manufacture the generators: PowerPal, Ampair and Rainbow Power all produce small power generators for fast flowing streams.

    I would suggest your best first step would be to talk to an expert who can tell you what you need to know: there is a company called Renewables First ( who can tell you what you need to know. Their web site includes a really good page that explains how hydro works and how you can calculate how much electricity you should be able to generate from your stream.

Dear Michael,
I just finished reading your book a few weeks ago, and I found it wonderful, thank your, its very rare for me to feel this way about a book, full marks. This has nothing to do with it, but I am an ex aircraft engineer, so did enjoy your project layout from begining to end.
Right, here is the story. I live in Japan, and next month we will have a new house started to be built, the roof will be the typical A frame construction with a 60 degree angle to it. The system is going to be put together by Sharp, and will be grid connected. Yesterday I saw the plans for the solar array, and sharp will cover the whole roof, the house will face south west.
I have not spoken to Sharp, because the house builder (Daiwa house) are of course organising everything, its also cheaper through them, but they can't answer my questions because its not their field. When I asked about the roof angle, I was told 60 degrees the best angle for solar, that night your book was on its way to me;) I was then able to give them an angle print out for each month. Anyway, I am sure you know the question that is coming, what is the use of putting panels on a roof that is not facing south?
I have the expected energy output for each array, front panels set one, set 2 and then the back side set 3, which seems to really push the values up. I am missing something, or have Sharp now invented panels that can sit at any angle?
Of course I am the customer, and do not have to accept it, can you give me your thoughts please.

Sorry I don't know the type of panels they will use. Just looking at the plans here, which are all in Japanese, so mainly guess work: the front of the house has a split roof, i.e. one is set back and this small roof has 3 panels on it, and will produce 449kwh the larger front roof has 11 panels on it, and will give 1656kwh the back roof facing NE will have 15 panels on it, and produce an average of 1.824kwh so the front array gives 2.1146kwh on average over the year and a grand total of 3.929kwh I see highlighted, and I think it says the govenment/electricity company will pay 48 yen/kwh.
     Dale, January 17, 2011

    Sharp are one of the best manufacturers of solar panels around today, but as far as I am aware, they have not manufactured a panel that can optimise energy at any angle, especially if it is almost always going to be in the shade.

    Professional solar installers do have access to some very expensive and comprehensive shade analysis tools that can sometimes produce some surprising results, suggesting locations that are not always obvious to the observer. For instance, depending on the exact angle of your house, you may well get a fair amount of sunlight on a North-East facing roof during the early hours of the morning - especially in the summer - and on the right tilt, that can be quite effective.

    However, you should ask to see the results of their site analysis. If they have not done one, you should be asking for one to be carried out. If they have done one, they should be happy enough to show it to you.

    Incidentally, I was involved in a project recently where we had to power a small retail outlet from solar panels that were mounted on an East facing wall and in almost constant shade. We managed to achieve it, although it was by no means the most efficient solar installation I have ever done.

Is there a difference in the angle at which sun rises and sets between Summer and Winter?
     Shakeel, January 18, 2011

    Yes there is. In the Northern Hemisphere, the sun rises further south in the winter than it does in the summer due to the 23½° tilt in the Earth's axis, relative to the sun.

    If you look at the Solar Angle Calculator on this site and select your location, you will be able to see the angles at which the sun rises and sets for mid-summer and mid-winter.

Hi, I'm wondering how to calculate kwh from insolation (kwh/m2/day) and a given area to be populated by solar panels in m2.
     Paul, January 19, 2011

    Multiply the wattage of your solar panel by the insolation to get an approximate kWh per day figure for your solar array. You can work out the area based on the efficiency levels of the solar panels: a 6% efficient solar array (typical figures for amorphous thin film panels) means a 60 watt panel is one meter square. A 15% efficient solar array (typical figures for a polycrystalline panel) means at 150 watt panel is one meter square.

Solar Panel require for charging 12Volt 100ah,12volt 150ah battery
     Dipankar, January 21, 2011

    Sorry - I need a lot more information than that to give you an answer.

    Fill in the Solar Project Analysis form here: and I'll send you a full report on exactly what you need to buy to achieve what you want.

I am going to Indonesia to install some solar powered pumps and chlorinator systems. The water use is greater in the mornings. Are there data on the effects of Eastward orientation of the panels to increase the "early" power production?
Great site! So glad I found it.
     Burton Northam, January 22, 2011

    Interesting idea. Is the plan to power the pumps directly from the solar panels, or via a battery bank?

    If you are going to power the pumps via a battery bank, you would be best to face the solar panels in their optimum position to capture as much sunlight as possible during the day, rather than just the mornings.

    If you are planning to power the pumps directly from the solar panels, check that the pump motors can cope with the variable voltage from the solar array. If that could cause problems, use an MPPT solar controller in order to maximize the performance of your system.

    In answer to your specific question, I don't have accurate figures for an eastwards orientation from your specific location. Depending on the exact angle, I would suggest that you would get between 65-80% of the performance from solar based on an South-East bearing compared to a true South bearing.

    I would recommend carrying out a site survey using a professional solar tool to identify the true performance of the system. The cheapest is an iPhone application that you can download from iTunes called SMA SolarChecker.

Hi Michael
Nice job on the book...It has helped a great deal with my planning of a grid tied system here in Canada. The question I have is when I am using the solar irradiance calculator, is tracking taken in to consideration at all? If not what kind of multiplyer should I use? I am planning a sytem that will track east to west with an option of vertical. I am not sure that the vertical axis is worthwhile but am obviously open to comments. What is the "sweet spot" range of a solar panel with respect to sun other words is it worth tracking if the angle is 45 degrees or less? I must warn you...this may not be my last question.Many thanks in advance Tom
     Tom Sommerville, January 22, 2011

    Thanks for your question. I have not taken tracking into account on any of the options on the solar irradiance calculator. We're planning an update to the software in the Spring that will include a full tracking calculation.

    However, you don't have to wait that long. Find out your latitude and longitude, then create yourself an account on the NASA solar web site and you can get the information from there:

    With regards to vertical axis solar trackers, I have no direct experience of these myself, but from what I am told they don't actually make that much of a difference, so long as you can manually adjust the angle of the solar panel once every month or so. Over the course of a day, I am told there is not actually a large difference in performance.

Trying to plan a solar park installation, I would like to calculate the space between the solar trackers in order that each tracking device (panels ocupy 5x4 meters on each device) do not shadow the others.
If you could give me an answer it would help me a lot in my plans.
Thank you in advance,
Ilias Goulas
     Ilias Goulas, January 28, 2011

    It comes down to trigonometry. You need to work out the height of each solar tracker, work out the angle of the sun in the sky and then work out the spacing from there.

    There is a trigonometry calculator available here:

    Side 'a' is the height of the solar array less the height of the tracker mounting poles, angle 'A' is the minimum height of the sun during the day. When you enter this information and press the calculate button, the distance between each tracker is shown as side 'b'.

Hi Michael - a very useful book thanks - ( regardless of the fact it was for the northern hemisphere.)
I am building a holiday home at the top of the South Island ,Marlborough in NZ.
Its interesting to see different opinions on degree of panel tilt.
Anyway my question is this - should I size my system to the KWH I need or keep it smaller because in NZ we get up to 30% better performance than that which the panel is rated at because of our clean clear air.
The next question is, Chinese made panels - they are cheaper here (40% less) than the big brands but I am wary of the performance specs and build quality . Your thoughts are appreciated
     Martin, January 29, 2011

    Thank you for your comments on the book.

    I would be very surprised if you get as much as 30% better performance because of air quality. Although I can accept that the air quality will make a difference, I would have expected this to only be a few percentage points.

    Most Chinese made panels are now built to a very good standard - far better than they were a few years ago. I have used cheaper Chinese panels on a few projects and have been very pleased with the results. So long as you are happy with your supplier and the warranty is good, they are certainly worth investigating.

    There is a standard for rating performance specifications on solar panels, which means you should be able to compare a panel from one supplier directly with the panel from another. Ask to see the detailed specifications for the panels you are looking at and compare them with specifications for a branded panel. In particular, take note of the temperature differences, as the performance of some panels do deteriorate more than others on a hot day.

    You will often find that the equivalent Chinese panels are slightly bigger and heavier than the branded panels.

Hello Micheal,
I would like a solar tracker. I need the angles for to do it. How can I calculate?
I live in Almería, Spain.
     Elisabet, February 1, 2011

    There are so many angles to take into account for a solar tracker. You have the angles from East to West to track the sun from morning until evening, the angle of the sun in the sky, and the spacing between multiple trackers to ensure one tracker does not shade another (if you have more than one).

    You can find out the tilt information and the angles of sunrise and sunset by using the Solar Angle calculators on this site.

    Solar Trackers are very complicated pieces of equipment. I would recommend you talk to your local solar supplier to get them to recommend the right equipment for you.

This probably a silly question- but am I corect in understanding from your solar calculator that if you have to have your panels on a fixed angle you put them at the winter angle as that is when you need the correct angle most? The project is in the Algarve in Southern Portugal where an angle of about 35 degrees looks about correct.They would face virtually due south.Many thanks, great website.David
     David Ross, February 6, 2011

    It really depends on when you want to use the power, but yes, if you need power during the winter months it makes sense to mount your solar panels to optimise power collection during the winter period.

Hi Micheal
I was wondering how accurate the wattage ratings on panels actual are. I have seen private reviews on panels that vary by as much as a fifth of the rating. Is this typical and do you need a fudge factor when figuring your total number of solar panel needed?
     Todd Waddell, February 7, 2011

    The wattage ratings on the panels are accurate, but for a number of reasons, you will never get the full wattage from a solar panel at any given time.

    For a start, the rating is based on the sun being immediately overhead on a clear day, with ambient temperature of 25 degrees celcius. If the sun is not immediately overhead, the wattage you get at that moment in time is less. If the ambient temperature is higher than 25 degrees celcius, the wattage you get at that moment in time is less.

    Then there are losses because of the solar controller or inverter, which can be between 5% and 20% depending on the type of unit used. And then there are losses from the battery - you never get 100% of what you put into a battery back out again.

    The losses can be calculated with reasonable accuracy and all this is described in the book. You can also see this calculation on the solar project analysis tool on this web site.

Where do you recommend buying individual cells in order to make our own panels?
     Jim Dwyer, February 9, 2011

    No! Do not do it. Building solar panels is a really specialised process. Most of the instructions on the web explaining how to do this are actually quite dangerous. Avoid.

    Here is some more information about why you should not build your own panels:

Michael....enjoyed your book, and am starting my first simple solar my garage and driveway. I live in Sonora, CA and get lots of sun, but am on the north slope of a hill so get filtered sun thru the trees behind me during parts of the day. Would an amorphous panel due better in filtered sun like it does on overcast days?...thanks, Jim
     jim lombard, February 9, 2011

    Yes, an amorphous panel does perform better in filtered sun. You do not say what voltage you will be running your system at, I guess it is probably only 12 or 24 volts, but if you have multiple solar panels, you will also get better performance if the solar panels are mounted in parallel rather than series. This means that if one solar panel is in shade and one is in bright sunlight, you are not compromising the power of the panel in direct sunlight quite as much as you would if you connected them in series.

How can I calculation solar irradiation for every hour i have annual temperatures for each hour for example (30 C).and what is Q=Ic*Ac is it Ic constand value
     adel, February 9, 2011

    There are two ways to calculate solar irradiation: using a hugely complex mathematical algorythm to work it out, based on your latitude and longitude, or sampling the information over a long period of time.

    The first method has the benefit that you can calculate a theoretical energy level for every minute of the day, but does not take into account local climate. The second method is more accurate, but makes it more difficult to calculate solar irradiance on an hour-by-hour basis.

    There are ways of creating a hybrid of both systems to create a predicted hour-by-hour calculation using sampled data and then applying algorythms to predict the hour-by-hour performance. I am still evaluating different algorythms for calculating this as some of them are more accurate than others.

    If you want to read up on the subject, there is an excellent initial introduction to the subject at the NASA website (link below). However, be warned, if you want to work out the calculations for yourself, you are committing yourself to several weeks of hard work.

Can I install multiple G83 (16amp)Solar PV systems on a single phase supply?
     Glyn Roberts, February 11, 2011

    Unfortunately not. The G83 limitation is a real problem at the moment and the industry is lobbying hard to get this changed.

    Some electricity providers are turning a blind eye to systems that are slightly over the 16 amp limitations by approving systems in the same way as smaller systems. The UK Government is saying that this is up to the electricity providers themselves.

    It is likely that there will be a change to the legislation in the next two or three months, but unfortunately, there are no guarantees.....

What are the units of solar radiance? I see kwh/m2/day. So, one may think you need to multiply the average radiance by total m2 for the project in order to get the total output. But, I see a formula for roi which doesn't take the area into account. Which is correct?
     Helena Thornton, February 11, 2011

    OK. You do not need to take the area into account because this is already taken into account in the formula (i.e. it's the m2 bit in the equation kWh/m2/day.

    The bit that is missing from this particular formula is the power generation capacity of the solar panel, shown in watts. For example, if you had an 80 watt solar panel, you can calculate the anticipated energy generation capacity per day by multiplying the kWh/m2/day figure by 80. This would give you the number of watt-hours of energy that panel would generate per day.

    That total figure is actually still theoretical, because you then have to add the losses from the total system, such as temperature, conversion efficiency of the solar controller/inverter and battery efficiency, all of which needs to be taken into account when looking at the overall system performance.

ref the solar pathfinder, given that 95% of installations will be on house roofs, does the tool give accurate results when used on the ground in front of ther roof?
     berni white, February 13, 2011

    No. You need to use the Solar Pathfinder where you are planning to install the panels. Shading analysis is very different on the ground compared to when you are up at roof level.

Michael, I have solar hot water system for year round usage, the ten panels will meet 95% percent demand in summer and about 40% in winter. What angle would set panels for maximum performance. Lat. 31.6 Lon. 111.0
Joe Frey
     Joseph Frey, February 13, 2011

    If you want best year-round performance, you want to tilt your panels to 60 degrees from vertical.

    I've looked at other angles in order to maximize performance during the winter months, but the differences are so small, there is no real benefit in that.

I want to instal solar street light and i'll like you to advice me on how to go about it.what voltage would you advice me to run on. 2.what is the effect of current on solar electricity
     Engr.Elijah E., February 14, 2011

    That sounds like an interesting project. It is difficult to give you an absolute answer without knowing more about what you want to achieve: how big an area you want to light, how many hours and so on.

    I'm not a lighting expert, so I am probably the wrong person to ask. You can buy off-the-shelf solar street lighting, so it may well be worth you talking directly to one of these companies.

    I do not have any contacts at these companies, but here are some links that you may find worthwhile looking at:

Hi, I'm working on a project in New England, outside Boston, and am interested in understanding how a single axis tracking system might benefit production. Do tracking systems generally increase production in poorer solar insulation areas enough to warrant the extra cost? And can they handle tough winters?
     Chris, February 17, 2011

    Single axis tracking systems do work, but to be honest, they rarely justify the additional cost. You will normally find it is cheaper to buy extra solar panels instead.

    They are robust and they do survive tough winters, but at the end of the day they are a mechanical system. Will they last 25-30 years like solar panels do? I can't answer that question, because I don't know.

    The real benefit of solar trackers comes in the summer months with the long summer days. In the winter they make a small difference, but not a great deal.

    Unless space is at a premium, I would recommend that you would be better off buying extra solar panels to increase production rather than install a solar array.

What are the Fitts rates on the UK feed inTariff are they £/kw installed pence per installed watt or What?
For example If I installed a PV system with panels to generate up to 2500 watt what wouls the fitts payments be?
PS Ijust bought your book
     Stephen Trott, February 18, 2011

    The rates are based on the number of kilowatts GENERATED. You are paid for both kliowatt production that you use and for production sold back to the grid.

    There is a brief description of the feed-in tariff here:

    I don't know where abouts you live in the UK, but if you had a 2500 watt solar array, you could expect to generate around 10kWh per day of electricity in the height of summer and in the depth of winter you would generate around 2kWh per day of electricity, with an annual average production of somewhere around the 6kWh mark.

    If you are retrofitting an existing house with a 2.5kW solar array, this means that over a period of a year, you could earn an average of somewhere around £2.20-£2.50 a day from a solar array.

I have a question that has been bugging me for a few days and I can't find* an explanation hope you can help.
I set up an experiment to find the optimum angle for a solar panel (I live in Guatemala though originally from the UK) and it pretty much coincided with your angle calculator for Puerto Cortez (Honduras) which is the nearest town on your calculator to my location.
However during this experiment ( used old 50watt monocrystaline panel charging battery as load notice that when the slightest shadow fell on the panel (even 1 finger's worth) the amperage dropped by 70%
Typical amps before shadow (1 Finger) 3.6A, with shadow 0.5A the voltage reading (13.8v) hardly changed though.
I would just love an explanation.
* Can only come up with the following speculation--- as the cells are wired in series in the panel the shadow fall on one cel and formed a blockage for the flow of current which would reduce the current--??
     Stephen Trott, February 18, 2011

    You are absolutely correct: the cells within a solar array are wired in series, so if you have a shadow across part of a single panel, it impacts the performance of the whole array.

Hello I have at spa/hot tub which holds approx. 1000ltrs water just wondering if you think it viable to incorporate a heater to the system to keep water temperature at a set temperature . the system has a 1.5kw heater witch switches on and off automatically when temperature drops 1 degree . full system running consumes 2.5kw (pumps and lights etc. ). Rather than having a 1.5kw heater switching on and off when needed I thought a smaller heater running continuously from free power might be worth doing even if it only helps a little. Any help or suggestions would be most welcome. Thanks for your time, Steve.
     Stephen Mellenthin, August 2, 2009

    Solar power can definately help you here... but I would not recommend solar electricity: you would be better off investigating solar hot water systems.

    Photovoltaic panels (which generate electricity from sunlight) tend to be between 6% and 15% efficient whereas solar hot water systems can be up to 70% efficient and tend to be cheaper to buy as well - at least in DIY form, especially if you spend some time shopping around for the best prices.

    A solar hot water system incorporating a thermal switch should go a long way towards reducing your energy consumption. Exactly how big your system would need to be would depend on a number of factors - specifically whether your spa is indoors or outdoors; what months of the year you want to use your spa; where abouts in the world you are based.

    Meanwhile, you can reduce your current electrical consumption by using energy saving lighting - I recently saw a swimming pool lit by LEDs which looked stunning and are extremely energy efficient.

Dear Mr. Michael Boxwell
Many thanks for the good book, but i have one question, which is in page 15 and 16. You wrote that when connecting in series, we add the voltages of panel together and add the power( in watts)of each panel together you calculate the max. amount of power and voltage the solar will generate, and in page 15 you wrote that 4 panels, each on have 12V and power 20W and when connect in series it will be 48V, 80W array. I think in this case, we have to connect in parallel with another 4 panels with the same V and W to get 48V and 80W.
Best regard
     Rasool Ali, August 4, 2009

    Thanks for the comments about the book.

    With regards to your question, when you connect solar panels in series, both the voltage and the power increase, whilst the current remains the same. When you connect solar panels in parallel, the power and the current increase, whilst the voltage remains the same.

    Let me explain another way, using the example of a 12v 12w solar panel (I'm using 12w because this equals 1 amp at 12v, which makes the maths easier).

    If you take one 12v 12w panel, your solar panel provides 1 amp of current at 12 volts. If you take four 12v 12w panels and connect them in series, the power and the voltage is multiplied by four - so you have 48v at 48w, but your current remains the same - 1 amp.

    If you then take four 12v 12w panels and connect them in parallel, the power and the current is multiplied by four - so you have 12v at 4 amps, but your voltage remains the same - 12v.

    I hope this answers your question.

Is it possible to incorporate PV panels into a south facing conservatory roof?
     Michael Churchman, September 2, 2009

    Yes it is possible, but of course that will cut down on the amount of sunlight shining into your conservatory.

    It also depends on the amount of panels you plan to fit to your roof. A large panel weighs in the region of 18kg and so there may be weight limits if you wish to fit lots of panels.

    To secure the panels to the conservatory roof, you would probably be best to bond them to the roof although it may be possible to have special mounts fabricated for the purpose.

    Your best bet would be to choose which solar panels you want to work with, get the physical specifications of the panels (height, width, depth and weight) and speak to your conservatory supplier to see what they recommend.

Is there an inexpensive way I can heat my two 16-gallon water tubs for my horses using solar?
     barb henderson, September 5, 2009

    Yes there is, but I wouldn't recommend solar electric for heating water.

    Instead, I'd recommend a solar hot water system. Depending on what your exact requirements are will depend on the best solution, but if you want to keep to a budget I'd either recommend a simple swimming pool solar heater (typically available on eBay from around £100/$150) or build your own from plans freely available on the web (start at

Just wanted to say thanks for your website. I just used your Project Analysis report and it really is excellent. I just wanted a rough guide to see if a potential project was feasible and approximate costs and the report did just that, plus provided some extra tips. If we go ahead with a solar installation, I will definitely buy your book.
One comment, any chance of adding more countries to the Project Analysis? Our planned project will be in Spain, so I had to find the Latitude for our area and then find a place in the USA at the same latitude. Which also meant that the prices given in the report were in US dollars instead of pounds.
thanks again, Rob
ps I found your site through A1 Business Forum.
     Rob, September 22, 2009

    Dear Rob

    Thanks for your comments. the publishers are adding new countries to the project analysis tool at the moment: by Christmas we should be covering most of Europe as well as India, the Middle East and parts of Africa.

    We will also be providing prices in a number of other currencies as well.

    Best Regards


What do you think about feed-in tarrifs?
     John Newman, September 26, 2009

    Now there is a big question. How long have you got?

    I question what the Government hope to achieve with feed in tarrifs in terms of overall carbon reduction. I also feel that with the proposals as they stand, the 'early adopters' who have already installed solar stand to loose significantly, which is unjust.

    However, from the model seen in countries like Germany and Spain, it is clear that a feed in tarrif this scheme will encourage the short term take up of solar for grid-tie applications. Whether that is the best application for solar depends on a number of factors, such as geographic location and how the local electricity is generated on the national grid.

    Personally, I believe the plan is flawed and a better solution would have been to boost the renewable obligation certification scheme (ROC) by improving the funds available and reducing the paperwork and administration for solar home owners to go through.

    This would also have been of much greater benefit for off-grid solar systems, which have a much greater potential to reduce carbon emissions over grid-tie systems.

Can you recommend any good videos on the web to help me get started with my solar project?
     Andrew Scott, September 30, 2009

    If you are interested in a very small solar project, then there are a couple of good videos around. For more complicated projects, however, the videos tend to be too superficial to be of any real use: the problem is that there is so much technical information, it is better to present that data clearly in a book rather than to try and show it in a video.

    If you just want a flavour of what solar energy is about, here is an excellent video from Green Power Science demonstrating a basic system:

Mr Boxwell,
I'm working on a school project for which I plan to design a solar photovoltaic system to power a small boat (this is one the projects that you talk about on Apendix E of your book). Do you have any additional information on this type of project? Any links, articles, etc would be helpful. I have found some internet sites that have some information on these type of projects but the info is very superficial and does not provide any tecnical details on the design.
Julio Hernandez
     Julio Hernandez, October 2, 2009

    My little boat is a small inflatable with an electric outboard motor. The motor is from a company called Minn Kota (other brands are Shakespeare and E-Thrust). I've had it for a number of years and have found it to be a very good little setup for inland waterways (lakes and rivers).

    The outboard is a 12v 100w motor. I have an 85 amp 12v leisure battery and charge it using a 20w solar array. This configuration allows me around eight hours of continuous use on a single battery charge, which is far more than I need. The 20w solar array is a bit too small - from a flat battery it would take almost two weeks to fully charge the batteries - but because I don't use the boat that often, this isn't a problem.

    Because I only have an inflatable boat, the solar panels aren't actually mounted to the boat itself and I charge the batteries up when the boat is moored rather than all of the time.

    All of the other solar boats that I have looked at in detail use a very similar setup to mine - outboard motor, large leisure battery and a small solar panel (plus solar charge controller and battery cut-off switch). You can also add a 12v battery charge gauge if you wish - you can find these easily online or at car accessory outlets.

    The total cost of my setup (electric motor, battery, solar charge controller and solar panels) has been comparable to the cost of a petrol-powered outboard motor on its own. For that I have a virtually silent powered boat with no virtually no running costs.

    I have no experience of converting a petrol outboard engine to electric power, nor do I have any experience of inboard electric boats. If you are specifically after information like that, I would recommend that you visit the Electric Boat Association web site at

Mr. Boxwell
Thank you for your prompt response to my previous question. Your answer was very helpful. As a follow up question, I was wondering how the motor speed is controlled? Does the motor you use have its own power electronics to control the speed?
Julio Hernandez
     Julio Hernandez, October 2, 2009

    With the electric outboard motor, there is a speed controller built into the outboard motor itself. If you were planning to build your own electric boat from raw components you would need to purchase your own motor controller, along with a potentiometer to work as a throttle and a three way switch to allow you to select forward, reverse or neutral.

    The best known make for motor controllers is Curtis Instruments ( and they can supply the motor controllers, the instruments and all the ancillary components you would require. You will require a different type of controller depending on whether you buy a brushed motor ('DC motor') or a brushless motor ('AC motor') - for cost and simplicity, you probably want to stick with a brushed motor design if this is your first electric vehicle.

    If cost is an issue, the cheapest way to get the electrical equipment you need would be to buy a second hand electric scooter from eBay - that will give you everything you would need except a solar panel and charge controller: batteries, mains charger, controller, motor and ancillary components.

I am planning a grid tie system/small battery back up system, I have come across some Suntech 200w 18v panels, do you see any problems using 18v panels?

     Paul, October 19, 2009

    No, there will not be any trouble using 18v solar panels. In order to charge a 12v battery, you need a higher voltage going into the battery in order to charge it up. 18v is fine, so long as you have a charge controller to stop the batteries from being overcharged.

    With regards to your charge controller, the most efficient controllers are MPPT (Maximum Power Point Tracker) controllers which will reduce the voltage coming from the panel going into the battery without reducing the wattage. Other controllers will reduce the voltage and truncate the wattage, which in effect means you are likely to loose around 25% efficiency from the panel.

How do you calculate the total irradiance for total solar spectra? I need to know the basic calculations for energy exoposed to an object
     Prashant Gharal, October 20, 2009

What is your advice regarding using a solar panel INSIDE a caravan while on a storage site.
     Mrs Kelly Henderson, October 27, 2009

    I do this with my caravan! It depends on what you want to achieve, but if you just want to keep the battery topped up (i.e. not recharging it from being flat) between uses, a solar panel will work reasonably well, so long as it is left in a south facing window and the panel is not completely shaded.

    I've got a 10w solar panel for this purpose (its around 30cm square) which cost me £35 on Amazon. It's small enough to prop up inside the window and big enough to provide a reasonable current - even during the winter months - to make it worthwhile having.

    I would avoid the very small panels (1.5 - 3 watt panels) for this purpose however as they simply aren't big enough to do anything worthwhile.

    If you want to do something a little more ambitious - like recharge the batteries from flat between uses - you would probably be better off fitting a larger solar panel to the roof of the caravan and installing a charge controller to ensure the batteries never get overcharged.

Hi Mike,
I just got your book today I bought last week through Great piece of work and easy to read. I will probably ask more questions in the future. For now, could you please provide solar irradiance in Nigeria (all states) and Cameroon (all main cities? Why you did not cover many countries in Africa?
Great job,
     Dr. Alain Nkoyock, November 2, 2009

    Thank you for your comments. I am adding new countries all the time - around 10 countries a month. I will update the web site with the solar irradiance figures for Nigeria and Cameroon in the next week. My intention is to cover the whole of Africa, but this will take a little time to achieve!

Hi Michael -
I came across your website after hearing about your book on I intend to order a copy next payday!
I have been to other websites that claim you can construct your own solar panels from materials ready available at local hardware stores. Is this true? If yes, do you address it in the book?
     Bob Jewell, November 6, 2009

    Thanks for your message. I know the web sites you refer to - avoid them. Visit and then ask whether the risks associated with home made solar panels are worth taking.

    Professionally built solar panels are dropping rapidly in price at the moment. You aren't going to save any money by building your own.

    You cannot build a solar panel at home to anywhere near the same quality of a professionally manufactured unit. At best your home made panel won't provide much power and will degrade rapidly. At worst it could end up being a fire hazard.

hi, michael I have a project that I could use your advice on, i have a 20x20 building
it does not have a hot water heater but it does have a small window unit and 2, 8foot for resident lights and it also has two plug-ins on each wall. my deep well pomp is a one horse motor.075kw that I would like to run also.
My question is, what type of inverter should I use? do they make inverter's four 220 outlets. In this building I have had the power company to cut my power off. I would like to run pv and also win power. This building is my office
so I would like enough power to operate this small build. my last KWH USAGE WAS 600
And also thank you for your book it is a good one, as you can tail I am just an amateur at this thank you and God bless.
     larry, November 7, 2009

    You can get 220v inverters without a problem - there are lots of different makes and models. I'd recommend a pure sine wave inverter and my personal recommendation for make would be 'Sunny Boy' - they're good quality, reliable units that won't let you down.

    You will need to work out your wattage use on a daily basis, as described in my 'Scoping the Project' chapter. This will give you a much better idea of what you will need. At a guess I would suggest that if you used 600kWh of electricity in your last quarter you are going to need to look at reducing this load first.

I am doing an assignmint at college on, reducing the average energy bill of a household. I have chosen solar power and have to go into great detail about what solar power is, how it works, optimum angles, light intensity at my longditude etc. two other solutions were required, i chose wind and solar-wind hybrid. any insight you could give on household solar power would be greatly appreciated. thanks
     Ryan Daly, November 10, 2009

    You've got yourself a big subject there! The first thing about reducing the average energy of a household is making sure you do the obvious things first - low energy lighting, low energy appliances, making sure everything is switched off if you're not using it, switching off equipment at the wall socket and not leaving them on standby.

    Once you've done that, you then need to look at house insulation - making sure your not loosing heat through windows, doors, walls and the roof.

    Then you need to look at your heating and hot water. You want the most efficient heating system available in order to keep your heating costs low. A solar hot water can help here, heating the water up that can be used for washing and - in some cases - for household heating as well (especially with underfloor heating systems, where excessively high water temperatures are not required).

    Finally, you can look at solar electricity to generate some or all of your electrical energy.

    Why do it all in that order? Quite simply, although the insulation and investigating the electrical usage are boring, its where you can make the greatest savings - an average household can save 30% of their bill by doing the simple stuff, and quite often for a very modest amount of money. All those savings can go towards paying for your solar equipment later on.

    I'd suggest you use the online calculators on this site, and the solar project analysis tool to help you get some of the core information you are after.

    Good luck with your assignment.

I have a photovoltaic system with battery charging and backup that uses TRAC inverters. The system is far too complicated for me to understand or deal with. When I have blown circuits during holiday festivities the battery backup is a failure and requires "resetting" which I, to this day, don't understand. I have no way of knowing how much my system produces and everyone, starting with the installer, says "well, you produced the difference between how much you used last year and this year". My system was inoperable for a 3-6 month period due to fried junction box on the roof that had filled with water. I had no idea it wasn't producing and no measurable way to know. What's the big mystery. Why can't I have a flow meter, just like the one coming IN from the grid, installed coming IN from the solar panels, so that I can see the volume of production. This system is 7 years old and I've spoken with 3 solar installers who all blow me off similarly. I'm a terrible salesman for the solar industry and I'm constantly asked "how's your system working out?" I'd love to be able to tell them. Can you answer this question?
     charles young, November 10, 2009

    First of all, congratulations on taking the step seven years ago on installing a solar electric system. I'm sorry to hear you've had problems with it - it does sound like you need somebody to have a look at your configuration. It does not sound right if your battery backup system needs resetting every time you need to rely on it, but without knowing the intracies of your system, it is hard to comment in more detail.

    With regards to the information you can get from your system, the technology has moved on in the past few years and most particularly it has moved on in terms of the information you get from the systems. There are two answers - the expensive one is to replace your inverter with a newer model. This will give you all the information you could ever want, but at a cost of a new inverter.

    If it were my money though, I'd go for a much cheaper solution and fit a standard utility electricity meter between your inverter and your load. This will give you a flow reading from your photovoltaic system into your home. Nice and simple to install and it will give you the basic information you want, plus change from $100. Some battery powered meters don't even need you to cut the wire, but clamp around it, which makes it really simple to install.

    You will be able to buy a suitable meter from your local electrical hardware store, or you can buy online from companies like

Are solar panels better than wind turbines
     Anonymous, November 11, 2009

    I answer this in my article 'Wind Turbines vs Solar Panels'.

    Unfortunately, there is no clear answer to the question - wind turbines can work extremely well if you have the perfect site for them - if you have a site with higher than average wind speeds, a clear air flow with no obstructions and no turbulance, wind turbines can be hard to beat.

    The problem is, very few people have the ideal site for a wind turbine, and if you don't have a good site for a wind turbine, you are going to be very disappointed with the performance of your wind system.

How much is it to get solar panels?

     autumn, November 11, 2009

    Prices are coming down all the time - a 5w solar panel can cost around $30 and capacities and prices go up from there.

    There is a Solar Project Analysis tool on the web site that can give you some indicative prices for your project based on how much power you wish to generate. I'd suggest you work out what power you need, then fill in the questionnaire on the project analysis web page. That should give you a reasonable idea of what your system will cost you.

Hi Michael,
I have not got a question as yet but know I will have soon but I just wanted to congratulate you on the best book I have ever read! Your style is so easy to understand so practical and so honest and as a heating engineer with a reasonable knowledge of solar for hot water and a good basic knowledge of electricity you are not patronising. Brilliant well done.
I lied about no questions, can you write a book, like this one, about wind turbines and other forms of power generation please.
Best wishes,
Martin Downes

     Martin Downes, November 12, 2009

    Thank you Martin, I really appreciate your message - I've had a tough day today and your comments have really cheered me up!

    A book on wind turbines is on the cards... but that is going to be a little while away yet: my next book is on electric cars, and that one is coming out early next year.

how many kilowatt hours per month with 30 200 watt sanyo panels with 3 2500 watts sma inverters in las Vegas with ajustment in tilt thank you
     Roger Cooley, November 14, 2009

    That is quite some setup! Assuming you have a site without any shading issues, I would expect an average daily figure of around 17½kWh +/- 20% at this time of year. That will drop to around 15kWh a day in December.

    During the height of summer, I'd expect you'd be generating around 37-42kWh +/- 20% a day, although because of the intense heat you get in the summer, you'll find you'll generate more power on cooler days than on the very hottest.

Hello Michael, thanks for writing the book, it's got a lot of useful advice in it and is written in a accessible style - Hooray!
My dilemma is two-fold; I'd like the government grant to help pay for the solar panels, so that rules out installing it myself (for a system just over 1kw, which is all I have room for, the grant is a sizeable percentage of the cost) and secondly I'd like a back up supply in case of a power failure. Page 137-8 (Appendix E) describes a back up system but it doesn't spell out if this works with a grid-tie inverter.
As a follow on to the second question, I have heard that in the US inverters exist that can send power back to the grid but which don't shut down when there is a power cut - I presume they have an isolator switch which stops the flow of electricity out of the house - can an isolator of some sort be fitted to a UK-style grid-tie system at a later date, if the system in Appendix E doesn't work with a grid tie inverter?
     Gary Miller, November 18, 2009

    Dear Gary

    Thanks for the comments about the book. You are right about the current grant system in the UK - you're going to have to get a professional installation in order to get access to these funds.

    With regards to a power backup system, there are grid-tie systems that now include a power backup system as well: in effect, these work like uninterruptable power supplies (UPS's) so that power to the building continues even if there is a mains power failure.

    You are also right about the US inverters that do not shut down when there is a power cut. As far as I am aware (and I could be wrong on this count), there is no such inverter yet available in the UK. Of course, a manual isolator could be fitted, but getting approval for this for a grid tie installation could be tricky.

Hello Michael,
I got your book for Christmas and it was the first thing I read.
Examples and calculations are excellent as is the total book & website content. Could not recommend it highly enough.
Just a quick question regarding the feed in tariffs.
We are being offered (relatively) high tariffs but the system compels you to use a Microgeneration Certification Scheme approved installer.
This means that you have to pay 'top dollar' for the installation, parts etc. with no choice of components at all. This outweighs most of the cost benefits of installing solar PV in the first place and really puts back the payback period.
Do you know whether any of the schemes are being expanded to allow inclusion of DIY systems? These are just as valid, especially as the final mains connection has to be done by a qualified electrician.
Thanks a lot.
Best Regards
Mike Norton
     Mike Norton, January 7, 2010

    Dear Mike

    Thanks for the comments about the book. It is much appreciated.

    Typical for the UK, the Microgeneratoin Certification Scheme insists that you use registered installers. Like the solar hot water systems this then means that instead of having a solar hot water system installed for under £1000, it costs you three to four times as much.

    The old scheme that they have phased out allowed for DIY installation, and worked extremely well. The new scheme that is now coming in penalises people who have already put in solar power and dramatically increases the cost for installing solar in the UK: I suspect the scheme will actually mean less solar power is installed in the UK rather than more.

    Solar electricity is a very simple form of power generation that is easy to install, so long as you follow some basic rules. Unfortunately, the bureaucrats have got it badly wrong, and as ever, the rest of us have to suffer.

    However, there is an answer. I think we'll see a lot more 'small scale' solar in housing from ingenious DIYers - powering the lighting system in a home, for instance, is a straightforward thing to do and can be done entirely independently of the main power in your home. It depends what you want to achieve - and how much you want to spend - but my advice would probably be to start with a small stand-alone system and then build that up over a period of a few years.

In my small off-grid system, I would like to provide the ability to charge the battery bank from the grid during prolonged cloudy periods. Is there a way to automatically do this without damage to the Morningstar MPPT controller?
     Emery Mitchamore, January 7, 2010

    There are controllers around that will allow you to do this automatically - but they aren't cheap.

    I don't know the Morningstar MPPT controller so I am not entirely sure what would be the best and most cost effective way of achieving this. I'd recommend talking to MorningStar to see what they suggest.

    Two options come to mind - both of which would require further investigation. The first is to install a small transformer to bring mains voltage down to an appropriate voltage and feed the power into the system as if it were a solar array. Check with MorningStar before you go this way.

    You would still need to manually switch the transformer on and off - you could partially automate it using a timer switch - but making a fully automatic system could be very complex - unless you are handy with electronics.

    The second alternative would be to charge the batteries directly when they run low. Again, check with MorningStar to see whether you would need to disconnect the controller if you went this route.

    Creating a manual transfer switch would be very straight forward, but an automatic transfer switch would be much more difficult: you would need to have another controller that is measuring the charge from the batteries, measuring the amount of power coming from the solar array and then switching across to the mains supply if the solar power and the batteries are both low, and then switching back again afterwards. If you're handy with electronics, it could easily be done, but otherwise I think you would struggle.

    So sorry - I can't give you a full and complete answer, but I hope that will give you something to go on.

Thanks for the great book (2010 edition). I need to build some grid fallback systems for a construction site. I was thinking about using an inverter/charger. As you probably know, these act like UPS systems where AC is the priority source. I was thinking about using an automatic switch so turn off and on the AC supply to the inverter. If the batteries deplete the Inverter/Charger turns on, charges the batteries and supplies grid power. If the sun comes out then the panels can aid the charge. Once the batteries have reached their full charge voltage (or some set voltage you have selected) the switch disconnects the grid source. I want to know if there is such a switch? And where do I find one? I think they are like a generator auto start switch but I need one used for grid source instead. Appreciate any insite or ideas you may have.
     Dean Sala, January 11, 2010

    There are solar controllers available that will do exactly what you want. Alternatively, all you are actually after is a relay between the mains supply and the battery charger, which switches on and off depending on whether your solar inverter is running or not.

    There is an excellent introduction to relays here:

    Depending on how your system works and how much electricity you need, your other alternative may be to fit a timer to the charge controller so that it only charges at night. This could be done either in conjunction with a relay or separately.

    The benefit of this is that you can then use cheap rate electricity (if you have a night tariff) and the carbon footprint of the National Grid at night is much lower than it is during the daytime. If you are based in the UK, you can actually monitor the carbon footprint of the National Grid in real time:

Hi Michael,
Can I ask - have you investigated the use of parabolic mirrors to focus light sources onto solar panels?
If so, can you comment as to whether this increases their power generation?
     Nathaniel Trevivian, January 12, 2010

    Yes I have, and yes it works... but proceed with caution.

    I have only experimented with parabolic mirrors for fun and never in a live installation. The issue is that although it increases the intensity of the light and thereby increases the power generation, it can also burn the solar panels and destroy them if you end up with too much intensity.

    I have experimented with parabolic mirrors in the winter months, and I have also experimented with them at night (to see how much power I could get from the moon!) and they can be quite fun, but I would not recommend them for a serious application.

Dear Michael,
Many thanks for the great book.
I have a quick question about inverter sizing for a grid-tied system. How close to the maximum power rating of an inverter is it advisable to go ? For example an SMA Sunny Boy 3000 has a max DC power-rating of 3,200W (, could I install panels with a nominal power rating of 3.2kWp (or even higher given that actual power generation is never likely to hit this particularly in the UK), or is it advisable to leave a 'safety margin' for the inverter to ensure maximum relaibility/lifespan - e.g only install a maximum of 3kWp of panels with a 3200W rated inverter ?
Many thanks
     Bill, January 13, 2010

    Dear Bill,

    Thanks for the compliment.

    Inverters are funny things. They're not so fussed about the power coming into them as they are the power going out of them.

    So you'll wear out an inverter quicker by using all 3.2kW of electricity than you will 'over feeding' the inverter in the first place.

    You really want to match your inverter to your solar array as well as you possibly can. The reason for going for a bigger inverter than your array can handle is really down to whether you can forsee wanting to expand your array in the future.

    As a rough rule of thumb, you should be aiming to use no more than 50-70% of your inverter capacity as 'constant' power - so your 3.2kW should be a peak rather than a constant load. Do that, and your inverter should last you a very long time.

Q.1 I would like to install a solar panel on my RV unit(recreational vehicle); Do you have some detailed instructions how that could be done specifically on an RV.
Q.2 I have read somewhere that the "Peel and Stick" solar panels are more efficient than glass panels, shatterproof, flexible and easy to apply.? What are your thoughts on these flexible panels? And especially how these would work on the roof of an RV?
Thank you,
Alex. Sztogryn
     Alex., January 15, 2010

    I have fitted solar panels to my caravan and it is very easy: use an exterior grade bond such as mastic to fix the panel onto the roof of the RV making sure you seal all the way around the panel.

    If you drill a hole in the roof to feed the cables through, make sure you also seal this with the same method.

    As an aside, make sure the panel you choose uses toughened glass. The vast majority of them do, but there are a few of the cheaper panels that don't.

    The peel and stick panels are shatterproof, flexible and easy to apply, but they are not as efficient as the glass panels. The panels I have come across have all been amorphous panels providing around 5% efficiency. They are great at what they do - they are widely used by river boats because you can even walk on them - but they are not cheap. For an RV, I'd recommend a glass covered panel every time.

Dear Michael,
I just read you book (2010 edition). I found it really good.
I am in Buenos Aires, Argentina.
I started a solar company three months ago and I need to find information about solar insolation in my region (Argentina, Uruguay, Bolivia, Peru, etc). I couldn't find any information of those countries in your website.
Could you help me with this ?
Best Regards,
     Maxi Postigo, January 19, 2010

    Of course I can. I shall update the solar insolation tables and solar angle calculators with information for Argentina, Uruguay, Bolivia, Peru and Costa Rica this afternoon.

    If you check the web site in two hours time, you should have all the information you need.

    All the best,


Dear Michael,
I am just reading your book and i wanted to simulate with your online calculator. I am from italy and there is not italy between the countries for irradiance and tilt calculations.
Is it possible to add?
     giulia suriano, January 22, 2010

    My apologies - I cannot think how Italy has been missed. I have got the data for Italy and will update the web site in the next half hour.

We are thinking about installing rooftop solar wall panels in order to prewarm outside air for a set of commercial air handlers. The installation requires multiple rows of solar heat panels and ducting. The panels in each row are approximately 3 ft high. We do not want the panels to steal sunshine from each other. Would you please tell me the mean sun angle for Seattle Washington so I can properly space the rows for optimal performance?
Thank you for your time and I look forward to your reply.
John Payne
     John Payne, January 24, 2010

    Thanks for your e-mail. 'Optimal performance' will vary at different points throughout the year. Are you looking to provide optimal performance during the winter months, the summer months or an average that will work throughout the year?

    If you want to achieve optimal winter performance, I would suggest an angle of 26 degrees from vertical. For optimal summer performance I would suggest an angle of 58 degrees and for all year round performance I would suggest an angle of 42 degrees.

Can you tell me what to spend for annual maintenance? Should I budget 5% of the system cost? What is likely to break? Other than break/fix, are there additional maintenance tasks that need to be done?
     John Parlapiano, January 28, 2010

    Annual maintenance is very cheap unless something actually fails. Excluding failures, budget $10 a year and an hour of your time every three months or so for regular maintenance. That maintenance consists of checking electrical connections, checking the batteries and topping up with water (if using 'wet' batteries) and making sure the solar panels themselves are clean and in good condition.

    The most likely thing to fail is the battery pack, although it is not unknown for controllers or inverters to break down.

    If you have a battery based system, you will need to replace the batteries at some point during the lifetime of the system: how long the batteries last will depend on your system setup - see the section on Batteries in the chapter on Component Selection and Costing for more details. As a rough rule of thumb, I'd suggest you need to budget for a complete change of batteries every four years if you're using cheap batteries or every seven years if you're using good quality batteries.

how much energy does an average home 1500 sqft require? in kwh
my bill says 856kwh in this period. do i average that by 30 then know the actual daily demand and from there determine the equipment sizes
     geo, January 30, 2010

    Yes - that is absolutely right. Devide the number of units of power used by the number of days.

    In the UK, the average home uses 0.8kWh of electricity each hour. In the US, the average home uses 1.2kWh of electricity each hour. Of course, these figures will depend on how you heat (and/or cool) your home, how many people live in your home and so on.

    To get the best out of solar power, you need to see how you can reduce your demand for power: it is more cost effective to halve your power usage than it is to buy a more expensive solar system.

Firstly, thanks for the book
I need some advice, and would be grateful if you could help.
I am beginning to design a PV system for a beach hut, which will power fridge, lights etc.
I envisage using a MPPT controller, and a 24v system, which I believe will be more efficient, however the recommendation seems to be to take the DC load from the MPPT. Whilst I would take the 24v fridge load from the MPPT, i would need to take the 12v light load direct from one of the two 12v batteries, would this be OK?
Thanks for your help.
Andrew Britten

     Andrew Britten, January 31, 2010

    You would be better off connecting two 12v lights in series rather than powering the lights from one of the batteries. Otherwise you will end up with an unbalanced battery pack which will reduce the lifespan of the batteries.

    If you would prefer not to run two lights in series, you can get a voltage converter to drop the voltage from 24v to 12v. These aren't expensive and can be purchased from any electronics shop or truck-stop (most trucks run on 24v and an adaptor to run 12v car accessories in the cab is a popular piece of kit).

Hello Michael i have a quick question for you..i bought your book off of amazon and its a great read so far..i have a photovoltaic installation class coming up monday and your book has helped me prepair...anyways i have the 2010 edition and it says its internet linked..could you send me the URL for the website? or explain how i can use the internet and my book together. thanks mike ttyl
     Hans Van Houten, February 4, 2010

    Dear Hans

    Thanks for your message. You've found the web site - The web site has lots of resources which will be useful if you are looking to put together a photovoltaic system:

    The 'additional resources' section includes lists of web sites, many of which change too rapidly to be worth putting in the book directly.

    There is a news section, which reports on interesting news in the solar sphere and new articles that I write on a regular basis.

    The most important part is the Online Calculators. These calculators allow you to look up solar insolation and angles for installations across the world - if I put them in book form there would be over 350 additional pages of information! Finally, the solar project analysis tool allows you to enter your power requirements and have a custom produced 11 page report detailing the configuration and likely cost of a photovoltaic system.

    Good luck with your installation class!

Hello there, bought your book and am doing some research on your website. I live in Melbourne Australia and was trying to use your calculator to get an idea of my system requirements. I got unstuck at the insolation figures - Can you change the website so that I can enter my latitude and insolation values if I know them (I do..). At the moment picking the "pre-canned" values for the UK, USA, Canada, India/Sri Lanka, etc doesn't help me at all. Thanks kindly. Matt
     Matt, February 6, 2010

    Dear Matt

    Thanks for your e-mail. I shall have a word with the web designers and ensure that Australia is added to all our calculators. I'll do my best to get this sorted as quickly as possible after the weekend.


I have a remote cabin that gets maybe 2 weeks of use during a year - one month max - in the summer. Usually there is 2+ weeks between <7 day uses giving plenty of time to recharge batteries via solar panels. We currently have propane lights, refridgeration and stove, so running out of power is not the end of the world. I would like to have better lighting and the option of not having to run a gas generator to vacuum, use power tools, etc. It gets extremely cold there during the winter (-30F or more). I did not see in your book how I would calculate the size of a solar array required for a 2 week recharge of battery power that would give me ~7 days of power. Any word of wisdom on things that I need to consider?
     John Simons, February 11, 2010

    That is an interesting project you have there. In essence, what you need to do is work out your power requirement on a daily basis when you are on site. You then need to calculate the size of the battery so that you have seven days of reserve in your batteries.

    The good news is that you'll need a solar panel approximately one third of the size of the panel you would need if you were using your cabin all the time, which really brings your costs down.

I'm looking at installing a solar pv system around 2KW to take advantage of the government feed in tariff and grant, before it disappears. My research leaves me wanting a system I can add to using distributed inverters, like the SolarEdge Inverters. Is there any equivalent system in the UK?
     Anthony Sumner, February 13, 2010

    I like what I see of the SolarEdge inverter system, although I have to confess I haven't seen one in real life. For larger solar PV installations looks like a great solution. The concern I would have is the cost - the extra efficiencies of the system are countered by the cost of the additional hardware and I'm not currently convinced that the system has an improved payback as a result.

    I'd like to be proved wrong though! There has been some clever thinking going into this system and any way of optimising power generation from solar should be encouraged.

    Unfortunately the SolarEdge system is not available in the UK and I am not aware of any other system that is available in the UK that does the same thing.

How many solar pv panels would be needed to provide 2.8 killowatts of electricty thank you
     Ian Antonio, February 15, 2010

    I need a bit more information before I can answer that! The size of solar array you need will depend on where you live and what months of the year you want to use your system.

    If you can tell me where you are based and what months of the year you want power, I'll let you know what size system you will require.

Hi there.
I'd just like to say that your book has been great in setting a few ideas firmly in my mind for my next project. I'm part of an expedition team set for Borneo, and we're planning on using solar power to enable us to use laptops and LED lighting, as well as charging camera equipment.
The section on irradiance simplified things for me and I now think I've got the numbers and the plan right in my head:
4x40w solar panels, should give us 640Wh of energy per day.
6 hours of 50w laptop use, and 6 hours of 20W worth of LED lights should take us to 420Wh of use per day, plus the camera equipment should be less than 50Wh roughly. We'll also have a small generator in case of emergencies or lack of sun.
I'm thinking adding another 40w panel would be enough to account for 10% loss using an AC invertor, plus another 20%(ish) from a charge regulator - and we can always be flexible with our laptop use too if we're using a bit too much.
So if there are any problems with this plan, it'd be great to hear from you! I just want to double and triple check everything. Any help would be appreciated and we'd love to credit you on our website. The blog is ready at and soon we'll have another website up that we'll use to give free resources to schools.
Thanks for the great book.
Kind Regards
     Dan Sargison, February 15, 2010

    Wow! What a wonderful project. I've enjoyed looking at your web site.

    My concern with the solar side of things is that you are going to spend a lot of your time in the shade out of direct sunlight, which would severely reduce the amount of power you can generate.

    In order to give you the best advice, I'd like to find out more about your project - I'll send you an e-mail with my phone number on it in a few minutes so we can discuss your needs in more detail.

Thank you for posting the solar irradiance on the website. The solar irradiance fluctuates all year round and therefore solar module efficiency is directly proportional to solar irradiance. However, could you tell us which period these solar irradiance covers?
Thank you very much
     Glenn Miniscloux, February 18, 2010

    The information on solar irradiance comes from NASA and is based on satellite monitoring. Solar irradiance measurements are taken every three hours, every single day. The figures shown on my web site are based on the average readings over an entire month, in turn averaged over 22 years of readings.

    On any one day there will be fluctuations in the amount of irradiance - often as high as 20%. Within an average day, irradiance figures change throughout the day, with the best figures at solar noon.

    If you know your latitude and longitude, you can see the underlying data for yourself here:

on the flat roof, can i install collector flat?
     Arun Bapat, February 23, 2010

    Yes you can. You will find, however, that you will have significantly less power from your system - most noticeably in winter, spring and fall. In the middle of winter, the difference can be as much as 50%.

To set up solar panels (probably foldable flex panels) to recharge an e-bike battery at 36Volts 10 Ah, what type of controller do I use? Do I need anything else? Is it easy to set up?
     Ellen, February 26, 2010

    An ebike solar charger is a great application for a small solar system. There are a couple of ways of carrying this out.

    If you are looking for a fixed system - mounted to the roof of a shed, for instance - your best bet would be to have two sets of batteries, so that one battery pack can always be charging whilst the other set is in use.

    If you are looking for a system to take with you, you'll find that the solar panels themselves are going to be too bulky - either that, or you'll end up with a very small amount of power generation that will give negligable benefit.

    The biggest issue is going to be finding a suitable charge controller that runs at 36v. The majority of controllers run at 12v, 24v or 48v and whilst some 36v controllers do exist, you may have to do some hunting to get one at a good price. MorningStar do an excellent MPPT controller that can handle 36v. However, these are really aimed at bigger systems and may not be cost effective. The Al-Samawa ASG6001SC controller is another option, which is much cheaper but can be difficult to get hold of.

    The actual wiring up is simple - you'll need to modify your battery box so you can plug the solar array into it, you'll need to connect three solar panels together in series to give you the voltage you need, and then you run everything through a solar controller so the batteries don't overcharge. With the right equipment to hand, that bit should only take you an hour or so.

    A few years ago, a friend of mine who doesn't own a car and uses his 24v bicycle to go everywhere, modified a bicycle trailer to build in a solar charging system into the lid of the cargo box on the trailer. From memory, he had around 80w of solar panels, charging a second set of batteries in the trailer. The combined battery pack gave him a range of about 50 miles and in the height of summer the system could completely recharge the entire pack every day.

    Good luck with your project. It sounds fun.

Hi I have just bought your book, very informative.
My question is:- How do you measure a gable roof from the ground to work out how many sq meters are available for mounting PV solar panels using just one side of the gable roof.

     John Bate, February 27, 2010

How come the solar angle calculator shows a greater angle from vertical in June than January. Surely the angle from vertical should be less in June as the sun is higher in the sky then ( i.e nearer vertical )? Doesn't seem right to me. Based on United Kingdom, Birmingham.
Cheers, - Nick

     Nick, March 5, 2010

    Hello Nick, thanks for your message.

    Are you mixing up your horizontals and verticals? In June, the panel needs to be closer to horizontal than vertical, so based on this there is a larger angle from vertical.

    The confusion may well be because many solar experts quote the angles based on a horizontal plane rather than vertical.

    All the best

Hi Michael,
I found your book very interesting. I understand that from April 1st consumers can sell excess energy back to the national grid, can you advise me of the tarriff please? Or point me in the right direction.
     Lance Feller, March 9, 2010

    Dear Lance

    You've always been able to sell your energy back to the National Grid, but now the Government has published new feed in tariffs for new systems that guarantee a good return over the lifetime of the system.

    This particular scheme is only available for systems that have been installed by accredited installers - DIY installations are not elegible. If you are interested, shop around for a good price: most accredited installers are charging a significant premium which wipes out much of the cost benefit of using them in the first place.

    You can find the details at

    There is also a good web site at

Hi Michael,
Great Book - working through a simple 'lighting'-'stand alone' system for a Hunting Hut in New Zealand - would you have the 'Solar Insulation Values' for New Zealand?
     Matt Coombes, March 11, 2010

    Fantastic. I have updated the solar irradiance figures to include New Zealand.

    You can find the updated solar irradiance calculator here.

    So far, I've only put in ten locations to start with (the ten largest cities in N.Z.) and I'm guessing there isn't much call for Hunting Huts in large cities! If you need a more accurate set of figures, let me know your nearest town/village or lat/long and I'll get it for you.

What is the most optimum fix angle for pv in Los Angeles California?
     Yildirim Arkun, March 15, 2010

    As you are aware, the height of the sun varies at different times of the year, so the comprehensive answer is that it depends on when you want to optimize power generation - during the summer, during the winter, or at an average throughout the year.

    If you have the potential to adjust the angle of your solar panel throughout the year, even better, but for most people that is not practical.

    If you want to optimize your energy production in the winter, you should angle your solar array at a 40 degree angle to vertical (i.e. 50 degree from horizontal). To optimize energy production in the summer, you should angle your solar array at a 72 degree angle to vertical (18 degree from horizontal). For the best average throughout the year, I would recommend a 56 degree angle from vertical.

    I hope that helps.

Good day,
I have recently purchased your book, and am slowly reading through it.
Yesterday I tried to locate South Africa, on your data, but could not find it.
Is it possible to give me some idea of the Solar Angles and Solar Irradiance for South Africa, Bloemfontein, 29° 36' 43.17" South & 26° 03' 42.94 East?
     Hannes Coertzen, March 17, 2010

    Thank you for your message. I will add South Africa to the online calculators later on this week for you.

     EVERALDO SILVA, March 17, 2010

    This is always a difficult issue: once you have some experience, finding a job in the solar sphere is easy. Getting started is not.

    Without knowing where abouts you live in the world, it is difficult to provide specific information. There are training courses you can go on to get you professional qualifications, including some PC based courses so you can learn directly from wherever you are. There is a basic free one available here:, which may be worth signing up for.

    If you have some money that you can use to buy a solar panel, a controller, a battery and an inverter, you can start working with the equipment for yourself - if you have a project that you can use the parts for, even better as you can then at least include this as work experience.

    Other than that, all I can suggest is keep trying. Good luck.

Hi, I live in Los Angeles CA,do you know if there some store in LA, to buy some Cell,Bat... or Solar Panel. Can you help on this please.
     Jimmy, March 17, 2010

    I don't live in Los Angeles so I have no personal experience of any local shops in your area. Real Goods Solar has a site in Los Angeles - look at their web site at

I have read your excellent book, and want to calculate the Solar insolation values. I live on the mendip Hills (Altitude 900ft)which is 20 miles south of Bristol (UK). The area is subject to low cloud, mist or fog on numourous days of the year. Can you advise or provide a link where I can factor your Solar insolation values for this area for these weather conditions. During the summer (July and August)periods we do experience quite a lot of rain and where here in the mendips we can often also be in cloud for quite a lot of the time.
Thanks in anticipation
stuart mcmanus

     Stuart McManus, March 21, 2010

    I have lots of happy memories of walking the Mendip Hills and I have been lost in low cloud on at least one occasion!

    You can get this information from NASA. Go to and create a new account. Then enter your Latitude and Longitude (I looked up 'Mendip' on and got the figures 51.126 and -2.65) and select insolation values and cloud cover. You'll then get the solar irradiance figures and a percentage of cloud cover for each month.

hello Michael,
How can I calculate the min distance between collectors to avoid shading of one over the other and as a function of the slope of the collector.

     Jose M. Gonzalez, March 23, 2010

    Measure the height of the top of the panel from the ground when placed at the optimum angle you have chosen - it doesn't matter if you use inches or centimetres. Then look up the solar angle from vertical for December for your area using the Solar Angle Calculator on this site.

    Then visit this web site:

    The height of the top of the panel from the ground is 'side a', angle 'B' is the solar angle from vertical. Enter this information and click on the Calculate button. The figure they provide for side 'b' is the distance you should place between your first row of solar panels and your second.

Hi Michael
Have your book and was trying to get a project analysis, live in Phoenix,AZ, the information I have is KW/day usage from my electric bills, but I can't use that with the form that is provided on site - any suggestions/solutions ?
     Dennis Pullaro, April 4, 2010

    The answer is to put a single reading for your whole site and put it down as one hour.

    i.e. if you are using 20kWh of electricity a day, put it down as follows:

      Device Description: HOUSE
      Voltage: MAINS VOLTAGE
      Power (watts): 20000
      Hours of use per day: 1

    However, I'll warn you now - if you are planning to simply move everything across without looking at what you can reduce, you're going to be looking at a very big bill to transfer to solar!

can a solar panel used in Central America get too hot ? what happens if gets too hot is bad for system ?
     David Ellis, April 7, 2010

    If a solar panel gets too hot the efficiency of the panel drops. Some amorphous panels have been known to be damaged by very hot temperatures but crystalline panels should be fine.

Hi Michael,
I read your blog article on Solar Generators. I want to make (or buy) a solar generator capable of running an A category ice cream freezer on a UK 240v plug. It uses approximately 18kw per hour. I have absolutely no idea where to start. How much roughly can it be done for? I want the green credentials but haven't got unlimited funds!
I'd appreciate any advice you have for me.
Kind Regards,
     Philip Barnes, April 19, 2010

    How big is this ice-cream freezer? 18kW hours of electricity is enough to run 24 homes on average!

    If you've got a big industrial freezer unit, then unfortunately I have to tell you that you're not going to be able to afford a solar system to run it - at a rough guess you are going to be looking at a bill the wrong end of £100,000. Technically it can be done, and there is a video on YouTube about a company who has done it. Have a look at it here to see the scale of what is involved:

    Small freezers are a very different story. I would recommend changing your freezer though because in solar terms, an 'A' rating is not actually that good.

    You can actually buy very, very efficient 12v freezers which are specifically designed to be powered by solar. They're not cheap - prices start from around £400 - but they are somewhere in the region of ten times more efficient than A+ rated freezers. The cost of running one of those on solar power is likely to be in the region of £250-300.

    There are several suppliers of solar freezers. The most popular is Sundanzer (

Solar angle and solar irradiance for Bangladesh are not included in the calculator drop-down box.Where can I find them?
Thank you for offering an awesome book for the beginners.Most of the urban housing are pencil-structure(Flat-roof) and quite dense.Power requirement is high in comparison to the available roof area.Will appreciate your views.
     Ahmad Elahi, April 25, 2010

    Bangladesh is currently not included in our solar calculators. I shall speak to our programmer and ask him to add Bangladesh to the system for you. If you check in a couple of days you will find everything you need.

    With regards to your power requirements, solar does require a large amount of surface area to generate electricity. Unfortunately there is no easy answer to that: either you need to find some more space to put panels or you need to reduce your power requirement.

I have a Xantrex hybrid system. My problem is not with that, but with the panels.
They are stationary roof panels rated at 4950W. Professional installer says they're at optimum tilt. Ten strings of three. I've watched them for several months now and have yet to see them exceed 4000W. Inverter output a little over 3K. Should I expect more?
     John Stewart, April 30, 2010

    You don't say where you are based. You will only find you get optimum power in the summer months when the sun is directly overhead. If you want, you can buy a solar irradiance monitor which will tell you the strength of the sun.

    There are some other things to check - obviously keep your panels clean as dirt does build up on the panels and will have an effect. There is the possibility that you have a poor connection or a rogue cable somewhere in your solar array that is increasing resistance.

Hi Michael
I recently bought the 2010 version of your book from Amazon, very clear and readable. I tried to fill in some electrical usages in the calculator but it didn’t give a very sensible result. This was because I didn’t fill in the details for each of the dozens of electrical devices in our home but tried to add them together and guessed (wrongly) the hours of us. It’s kind of difficult to know how many hours they run as so many devices switch off or go on standby most of the time.
Can I do it a different way? I know my home consumes 5,350 kWH per year or 14.625kWH per day on average which excludes heating. So I know we use an average of 0.61kWH each hour. I was thinking about a ground mounted array of say 22 Mitsubishi 180Wp PV panels feeding in a grid tied system via an inverter.
What would that set up typically cost and how much of my annual usage could I expect it to cover?
Norwich, UK
     Ray Spreadbury, April 30, 2010

    I appreciate the difficulty in calculating the electrical requirement for each device in your home. The reason for suggesting that this is a good approach is to help you evaluate each device individually. For instance, some PCs use twenty times as much power as others and if you find 'power hungry' equipment when you audit your house, you can start seeing straight away where you can make savings.

    For a 'quick and dirty' approach, the best way to use the calculator would be to enter 14625 watts for one hour on the solar calculator - that would give you the figures you need.

    If you shop around for the best price and you're doing the work yourself, I would suggest a system with 22 Mitsubishi 180Wp panels would cost you in the region of £10,000.

    In the depths of winter, I would anticipate that system providing an average of 2½kWh per day, whilst in the summer months, the system would generate 17-19kWh per day.

    I would anticipate your suggested system providing pretty much all of your energy for around 5 months of the year - from mid April until the middle of August.

I have been trying to figure out if I can use a $300 inverter purchased at a store like Canadian Tire but nobody at the store knew. I called Sonny Boy and compared a similar size inverter at $3000 but they couldn't explain what was special about their inverter that would make it required for a grid tie. Have you ever done a grid tie and what inverter did you use?
     Darren Anderson, May 18, 2010

    All you are getting for $300 is a basic inverter that you cannot use for grid tie: for grid tie you needs lots of other clever electronics to synchronise your power with the grid's. A lower cost inverter is fine for powering stuff in your house though. Check that it is a pure sine wave inverter and that it includes a voltage cut out so that it doesn't completely kill your batteries.

I saw an article in Ottawa Metro paper which quoted you as saying solar panes cost 150 per. At that point I'm interested
When I go to one of my local suppliers, they suggest they are around 600 for a 120 v panel (Qualicum Beach BC).
So where are you getting your panels from (manufacturer or retailer). Do you recommend one manufacturer over another?
In my case I'm looking to reduce my electrical consumption from BC Hydro. I run a seasonal house and heating is all electric. What total costs should I expect as an entry point? What about solar versus wind? Maintenance is an issue as no one is in the house
9 months of the year, yet I maintain heat to prevent mold.
     Barry Morton, May 20, 2010

    Solar panels are dramatically dropping in price and in the next few years you can expect to pay much less for them than you do now. CAN $600 is pretty expensive for a 120w panel though. If you shop around you should be able to get one for under CAN $400. The next generation of solar panels are likely to be an order of magnitude cheaper, however. A 120w panel in a couple of years time could cost as little as CAN $50-75.

    I'm based in the UK, so I get my panels from different sources: I tend to go for Kyocera, although more recently I've been experimenting with the lower cost panels from China which are giving near-identical results for a lot less money.

    Electric heating consumes a lot of electricity and for that reason, you will probably find you'll need to find a different heating system for your home - especially as you have the heating switched on throughout the year.

    A solution that might work well for you though is a combination of wind turbine and solar panels, using an electric heater as a 'dump load' for when your batteries are full, thereby providing a small amount of heat in the house when you are not around. I've written an article on solar vs wind (you can find it under the SOLAR NEWS AND ARTICLES section on the web site) which will give you a basic grounding in that.

    Without knowing a lot more about your project and your energy consumption, it is not easy to give you a real indication of what it is going to cost you. There is an online project analysis tool that will work out some estimated figures for you based on your location and energy consumption on this site - visit the ONLINE CALCULATORS section and follow the links to the PROJECT ANALYSIS TOOL.

I have a basic photovoltaic question. If a solar panel is exposed to the sun, but the solar panel is not part of a circuit (not hooked up to anything), where do the electrons caused to move within the solar panel go? Do the electrons go into the atmosphere, or do they somehow remain in the panels? Please elaborate.
     Charles, May 22, 2010

    The energy turns to heat.

I find this site very interesting, however I have an issue with the solar angle calculator. It lists cities for every province and territory except Alberta. Since I live in Alberta, I find this inconvenient to say the least. Any idea as to why it is this way and will it be corrected in the future?
Thank you for answering my question and this very interesting site.
     Joe, May 23, 2010

    You've found a bug in the system. Oops! My sincerest apologies. We're working on a fix now and it should be working by the end of today.

Hi Michael,
I just finished reading your book, it was an excellent introduction to Solar power (especially all the information on sizing of the systems energy requirements), Thanks.
I apologize its a long mail
I have three questions though;
1. In certain equatorial parts of the world, a fan (ceiling, table or standing) are an absolute necessity, and they would normally run at least 12 hours a day. If I am careful I could choose a model which only consumes say 75Watts, in 12 hours this will require 900 WH of energy! If I use an 80W polycrystaline panel, and the minimum daily insolation throughout the year is about 4 (Port Harcourt in Nigeria), this means that each solar panel can fetch about 4 x 80 = 320 WH of energy daily, should come down to 85% of this if battery and inverter inefficiencies are considered, so I get about 320 X 0.85 = 272 WH of energy per panel per day. This implies that I would need about 3 solar panels to support each fan! What would you normally do when you have to support ceiling/table fans please?
2. From the insolation app on your website, the lowest daily insolation for the Port Hatcourt in Nigeria is approximately 4hrs. There sure seems to be longer hours of sunlight than this, there sre probably lots of rainy days during these period. In your experience how accurate would you consider the daily insolation information?
3. Have you every heard of "HQRP solar panels"? Their monocrystaline solar panels appear to offer more power per cost of panel. But I suspect they are probably cheaper because they might be inferior to Kyocera and Sharp. Do you know how they stack up to the top brands please?

     Chizim Eleyi-Okpara, May 24, 2010

    Thanks for the questions and for the comments about the book.

    There are such things as solar extractor fans that are designed to extract hot air from a room. These use very small motors using only 3-4 watts per hour. I've asked a couple of my contacts about proper room fans that can be run on solar panels - no responses yet, but I'll keep looking and let you know if I find anything.

    With regards to the insolation figures, they've been provided by NASA based on satellite observations. They take solar insolation readings from the surface of the earth every three hours and the data is based on a 22 year period between 1980 and 2002, taking into account average weather and solar angles at different times of the year. NASA say the figures are accurate to within +/- 20% on any one single day, but provide this average figure over a period of an entire month.

    The figures don't say how many hours of sunlight you will get in a day, but the total irradiance of the sun over a period of the day - in other words, you may get ten hours of sunshine, but in terms of irradiance it is the equivalent of four hours of noon-day sun.

    HQRP is a brand of solar panels that I have come across, but never used. Here in the UK they are not particularly cheap. Looking at the specifications, they look similar to a lot of panels built in China. So long as the warranty is as good as they claim (25 years) they would strike me as a good budget choice.

    There is less and less to choose between the cheaper panels and the top brand panels now. Quite often the cheaper panels are built on the same production lines as the top brand panels, but are rebranded and downgraded to a lower watt rating if there are a higher number of defects found at the end of the manufacturing process: so you might find a 200 watt big brand panel might only produce 195 watts at the end of the production line test and therefore sold as a 190 watt panel from a 'non-brand' company.

Hello. Iam a nigeria and wish to have solar irradiance(sun-hour) of some major Nigerian cities to assist me in designing solar electricity systems. how do i get your solar electricity handbook?
     MUSA NUHU MADAKI, June 6, 2010

    I will ensure Nigeria is added to our list of countries this week. You can buy the Solar Electricity Handbook from most bookshops, although you will probably have to order it in specifically.

    If you ask for ISBN 978-1-907670-00-8 and give the full name of the book (SOLAR ELECTRICITY HANDBOOK - 2010 EDITION), any bookshop should be able to order it for you.

    Alternatively, you can buy it online from The Book Depository, an English bookshop who will deliver it free of charge to Nigeria.

Dear Boxwell,
Your book and the website attracted my attention because I wish to do a solar project in the PHILIPPINES.
However, when I look at the online calculators, I noticed that Philippines or any other South East Asian countries are not in the list. Is there any reason for this?
I'm already convinced of the usefulness of your book but I am quite surprised not to be able to use the online calculators.
Looking forward to hearing from you.
Jonathan Mendoza
     Jonathan Mendoza, June 7, 2010

    Thank you for your message. The Philippines will be added to our list of countries that we include in the next week.

    All the best with your project.

You are doing a good job here and please keep it up.
I was wondering why Calabar, a city in Cross River State in Nigeria is not included in the calculation table you have prepared.
Could you do something about that please.
In the same vein, Africa is not captured in the your solar project analysis table.
Finally, could you advise if I am building a house that has a power consumption of about 23kw could be powered by solar and at what cost?
Alph UK June 2010
     Alph Takon, June 9, 2010

    Thank you for the comments about the web site.

    I will ensure that we update the Nigerian data on our system this week. We will be doing some updates to the Solar Project Analysis table in around two weeks time and Africa will be included on the next release.

I want to run solar fans to cool my horses stall.How can I do it?
     patti, June 19, 2010

    You can actually buy solar extractor fans as complete kits. Have a quick look on Google for 'solar attic fan' or 'solar extractor fan' and you'll easily find them. The question is, what do you actually want to achieve?

    Do you want a fan cooling system to blow cool air at the horses, or do you want a system to extract hot air from the top of the stall which will pull in cooler air into the bottom of the stall? Do you want a system that can run at any time - including the night, or are you just something for the daytime?

    A daytime system, or an extractor system, is the easiest - simply buy a solar panel and a 12v fan - you don't need a battery because the system will be self regulating. When the sun is shining, the fans run fast, when its a dull day, the fan won't run at all.

    Alternatively, if you want a system that can continue to provide air movement into the evening and night, you'll need a battery and a solar controller that switches off when the battery charge gets too low. Then you can have a system that will continue to run into the evening and when the battery gets too low, the fans then automatically switch off - by which time, with any luck, the ambient temperature is cool enough for your horses anyway.

I purchased the book through Amazon but it has not arrive yet fro 1 month now. so I decided to read the preview and utilize some resources to familiarize myself with Solar PV. When I used the Solar Angle calculator, the Philippines is not included. Are there plans to include that in the future? Or is this included in the book?
Thank you.
     Lucky Hormigos, June 25, 2010

    The Philippines is not yet included, but I will speak to our programmer and arrange for it to be added to the list of countries to include in the next week or so.

    Sorry to hear that it has taken so long for your book to arrive from Amazon - they usually keep my book in stock for very prompt delivery. If you still have not got the book, you may wish to cancel your order and reorder from as they will offer free delivery and usually dispatch within 1-2 working days.

Hi Michael... Firstly, your book has provided me with the most answers in the simplest of languages for a complete novice like myself. I now have so much more confidence that previously and am well on the way to coming up with a final design for my isolated house in Portugal.
I think I have decided on a 48v system, using a 3750w/5000w peak inverter. What would be the minimum quantity and spec of batteries and panels to get this system working, at first with low loads planning on expansion as time goes on? I'm guessing that I will need 4 batteries and 4 panels, maybe min 60w a piece?
Many thanks, Kevin.
     Kevin North, June 27, 2010

    Thank you for your message and for your comments about the book. They are much appreciated. Apologies for the delay in responding as I have been away.

    The number of batteries and panels you require really does depend on your overall load on the system. If you are planning to expand your system in the future, I'd recommend that you over-specify the batteries now so that you don't have to extend your battery pack in the future when you get more panels. Mixing old and new batteries later on down the line does reduce performance.

    You've got quite a big inverter there, so you're going to be pulling a drain of around 100 amps peak on a 48 volt system, so I would suggest that you want a minimum of a 200 amp-hour battery pack as a minimum: even a 200 amp-hour battery system will only give you around 2 hours worth of power at peak usage, so you may want to consider more.

    My personal recommendation would be 6v Trojan deep cycle batteries for that sort of application as they can easily withstand that sort of load for extended periods of time.

    A 240 watt solar array made of four 60 watt panels generate between 400 watts-hours per day in the depth of winter and 1.8kWh per day in the height of summer. If anything, that seems a little low, although of course that does depend on your actual usage is.

Greetings Michael,
I recently purchased your book to assist me in learning and setting up a small solar electric system at our remote cabin the Adirondack mountains for NY State. My system has meager need and will only be running some lights and maybe a radio. I have used your solar analysis calculator (very nice I may add) to calculate our battery needs at 90 Ah. I wound up purchasing a 110 Ah AGM battery just as a precaution. I will be charging the battery with an inexpensive 45 watt solar array (Chicago Electric). My question is this, can I store my battery and charge controller in a (slightly modified) cooler instead of purchasing a commercial battery enclosure? I have read that the enclosure should be weather proof and insulated to protect the battery from temperature extremes. If you think this would work should the cooler be ventilated even though I am using a AGM battery?
I would like you opinion on this idea.
Best Regards,
Christopher Cooper
     Christopher Cooper, June 29, 2010

    Dear Christopher

    What a great project! A cooler box would make a great battery box, but yes, you will need to add some ventilation holes into it.

If I was to conduct some maintenance on some solar PV panels and wanted to record the improvements in efficiency how could I calculate the result? I have a Solar Power meter (W/m^2) and can review the watts on the internal house meter. I was thinking if I could record the irradiance value and the internal electric meter (watts) before and after the maintenance (Cleaning) I could show the improvements. If it was a cloudy/sunny day and therefore the irradiance changed frequently just reading off the internal electric meter before and after would not be a fair result. So how could I factor in the reading from my Solar power meter (W/m^2) to achieve a fair result in efficiency?
     Rich C, July 4, 2010

    I'd recommend a few different approaches in order to get an accurate picture. Firstly, I'd measure the average watts-hours over the previous few days before cleaning and then again afterwards - as hopefully this would counter any minor differences in weather conditions.

    I would also use a handheld watt meter to measure the difference immediately before and after cleaning.

    You should get a fair indication of the difference using these two different techniques. From my experience, grubby solar panels have a 10-15% impact on efficiency, so it is definitely worth keeping them clean!

Hi,i have seen your review of a 12v 12watt panel on maplin's website, i was wondering if you could give me an idea of how much this would charge my 110amp leisure battery assuming good light. I am not particulally well knowledgeable on watts,amps etc.The method i use to give me an indication of the state of the battery is just by using a voltmeter ie... 13v = fully charged and 12v = discharged.
So how long would it take to raise the voltage by 0.1 for example using this panel ?
Hope all that made some kind of sense !! many thanks
     Steve Holcroft, July 5, 2010

    The particular panel you are referring to is great for maintaining a battery charge on this size of battery, but not big enough to charge it up from flat. I use two of these particular panels to charge up an 75 amp-hour battery on a caravan, and this will charge the battery from flat to full in around 7 days during the summer months. Because I rarely use more than half the battery's charge, that works out to a recharge in between 3-4 days in the summer and a week during the spring and autumn - which is perfect for the sort of use I use the caravan for.

    So one panel isn't really enough for a 110 amp-hour leisure battery unless you're only using a small amount of the battery's capacity. I'd recommend two or three, or preferably go for a higher wattage crystalline panel which will be a similar size to the bulky Maplin amorphous panel.

I have installed 1.5 Kw solar PV plant in my office in India.I want to know how long (hours)I can use the la loads i.e. FAN (75 W)x 5; CFL :18W x 15; Fridge
     Sumit Roy, July 9, 2010

    You don't say where abouts in India you are from, but from the sound of what you've got, it sounds like you have a big enough system to achieve what you need: your system should generate between 6kWh-9kWh of energy per day.

    Your fans are using 0.375kWh for every hour they are running. Your lighting is using 0.270kWh for every hour that is running. You don't say what sort of fridge you have, but it is likely to be in the region of 0.060-0.100kWh for every hour that it is running, unless you have a special low-energy solar fridge in which case it is likely to be around 0.010kWh per hour.

    So you have enough power to run your fridge 24 hours a day, plus run your lighting and fans when you need them.

Hi Michael,
I am here in Brisbane, Queensland, Australia - over the last few years the cost of power has sky-rocketed with the current State Government the sole cause. I could go on at lenght but that is not the reason for my email.
Anyway, I have been researching how I can utilitse the use of solar power to supplement or if possible replace our grid feed for our house and pool usage. From the snippets on the web I have seen of your book, I was wondering if there are any issues i.e. relating to conversions to what we use here in Australia?
     Ivor, July 10, 2010

    There is no particular issue in using solar power in Australia, and the amount of sunlight you get there makes using solar all around the year a practical solution.

    You can start small and grow your system as you like: a very popular solution is the grid-fallback system, so your home runs on solar power when you have power available and when you've run out, the system switches straight back to the grid.

I have a simple question and I would appreciate your honesty. I want to know how much money I would save if I built a system like this as oppose to buying the one you see in this link.
I have time to learn and do the project but if the saving is enough I will just purchase one. Money is tight right now and I do won an RV that needs this much solar power to charge by my large batteries that I now charge at home with an electric chord to my house outlet.
     Tony, July 11, 2010

    It is difficult to advise precisely without knowing your exact energy requirements and where in North America you're based.

    However, the specification of that Mr. Solar kit look good, it should provide enough power for most people's needs and it is a reasonable price.

    If I were you, I'd just buy the kit! Yes, you could save some money if you read my book and work out all the figures for yourself, but probably not enough to make the extra time worth while.

Recently spoke with a manufacturer that suggested using 185W monocrystalline modules on a 2-axis tracker rather that 280w polycrystalline modules. What is the reason for this? Is the difference in power generation so noticeable? Furthermore, if given the option, would it be best to choose monocrystalline over polycrystalline in any type of installation (tracker or fixed mount)?
     Ramses Torres, July 14, 2010

    The benefit of a tracker is that the panels track the movement of the sun across the sky. It depends on where you live, but a good two-axis tracker can effectively increase the power generation by around one third during the summer, and a little bit more than that in the winter.

    However, all that is offset by the cost of the tracker. If the tracker costs more than around $300 (and most of the ones I've seen cost a lot more than that) then often the benefits of the tracker is outweighed by the extra cost - you may as well just buy extra solar panels.

    That is not always the case, because you also have to have space for mounting the panels and so on.

    Traditionally, mono-crystalline panels have been more efficient than poly-crystalline panels, which effectively means the mono-crystalline panels are slightly smaller than the equivalent rating poly-crystalline panels. However, that is often not the case any more and many solar panel manufacturers now claim their poly-crystalline panels are just as good as monos.

    Interestingly, most of the new generation of solar panels now being developed are all either amorphous or poly-crystalline panels, capturing a much wider spectrum of light. I suspect the days of the mono-crystalline panel are numbered.

Hi Michael!
Very cool tool you have to calculate solar radiation. I was wondering what the equations you used to calculate the solar radiation and whether you would be willing to share with me the code! I'm a grad student and i'm new to modeling - i'm trying to model the amount of solar radiation in the photosynthetically active radiation band (or just a given wavelength) as a function of cloud cover and azimuthal angle but i'm running around in circles trying to calculate this.
Thanks a lot!!! Awesome inspiring work!!
     Heather, July 15, 2010

    Thanks for your message. Unfortunately I am not at liberty to share the code for my solar calculators as this belongs to the publishing company and has been created at quite some significant expense. However, you could do a lot worse than to visit the NASA solar site to get your hands on the raw data and to find out the calculations that they use.

    Have a quick google for 'NASA SOLAR TOOLS' and you should find what you are looking for quickly enough.

Hi, I have a roof line that faces 249 deg SW. The pitch of the roof is 35 deg. If I angle the solar panels aiming towared true south would I have to also increase the angle towared true south also? The location would be in the Southern New Jersey Area. Thank you Jerry
     Jerry Duffy, July 21, 2010

    The 'perfect' angle for solar panels changes throughout the year and working out the optimum for you depends on when you want to optimize the panels - do you want to generate as much power as possible during the winter, or do you want to maximize your generation capacity in the summer.

    35 degrees is actually pretty good for New Jersey for all-round power generation, giving you good generation capabilities throughout the Spring, Summer and early Fall. If you wanted to optimize late Fall, Winter and early Spring, you may want to increase your angle by around 8 degrees, but quite frankly, unless you want to squeeze every last watt out of your panels, you're pretty good on what you've got.

My caravan is kept in a barn. What maximum distance between the panel and the battery could a solar panel cope with? What size panel and who from?
     r wilkins, July 22, 2010

    I guess you're looking at a 12v system. Go for the best quality solar interconnection cables you can and aim for a maximum distance of around 30 feet/10 metres.

    As for the size of panel, that really depends on how much power you actually use. In my caravan, we used around 40 watt-hours per day on lighting, 100 watt-hours per day on a portable TV and 50 watt-hours per day on the water pump - a total of just under 200 watt-hours.

    We only needed to provide this power for six months a year so we needed a 50 watt panel to cover our usage.

    However, your requirements will be different to mine. Go and look at the 'Online Solar Calculators' and use the Project Analysis tool to find out exactly what you need.

Hi Michael
I'm an electrician just getting into solar. I am currently looking at a stand alone system for greenhouse/shed. the simplest plan is 3x 12v caravan style lights powered from a 110Ah leisure battery. however, I would also like to provide a price for a system using an inverter to create 240ac so that an anti frost heater (about 500W) could be installed. I have calculated that the batery will give me about 20 hours running time. what I am not sure of is how long the battery will take to recharge itself. I am looking at a small panel of 10W or so, do you think this will provide sufficient recharge? I would estimate 3-4 hours use of the lights and up to 12 hours use of heater (in winter when of course insolation is at its lowest!!)
According to my calc on a 10w panel in dec itwould take several days to recharge, meaning in a cold spell the heater will ceaseto function after a couple of days.
would appreciate your views
     Andy Thompson, July 24, 2010

    Hi Andy

    Heating is really not a good idea with solar electricity. Electric heating uses an awful lot of electricity.

    Is your 500 watts an hourly or a daily figure? Either way, it is a lot for a small system to generate. You don't say where you are based, but if you are based somewhere in the UK, and 500 watts is a daily figure, you'd need around 600 watts of solar panels to generate that amount of energy in the middle of winter.

    There are some other ways to achieve a similar effect. Here is a link to a really good article on solar heating in a greenhouse using a heatsink:

Solar panels are rated at STC values which makes sense to achieve a standard for comparison purposes but where irradiance levels are well above 1000 w /metre squared they surely produce a higher output. While the voltage remains sensibly the same I have measured higher currents than the specified Imp so what are the limits - does the pn junction become saturated? So can a 180w module produce more than the rated value?
     Peter Vowles, July 25, 2010

    Many solar panels can generate a higher output than their rating in optimum conditions, but in a real world environment they are unlikely to do so.

    Solar panels produce less energy when they are hot. When irradiance levels are above 1000w/metre, the high temperatures are reducing the efficiency of the solar panels.

    So what you gain in one hand... you lose in the other. Sorry!

I think your solar angle calculator on this page is labeled incorrectly.
I think those angles should be labeled "Figures shown in degrees from HORIZONTAL"
NOT "Figures shown in degrees from vertical"
     Bruce Schena, July 28, 2010

    The figures can be read in two ways. The way I meant it to be read is the angle of the SOLAR PANEL is from the vertical.

    However, it is also true that the figure is the angle of the SUN from the horizontal.

    So I guess I need to put a clearer description on the panel to avoid this confusion in the future. I shall speak to my webmaster.

    Thanks for the comment.

Thank you for writing a book about this subject.
It makes the subject a lot more accessible!
I just started reading the book, and I wanted to find out what the solar irradiance figures are for the Netherlands, but they are not mentioned in the country list.
Do you know where I could find them?
Thank you!
     Anna, July 31, 2010

    Thank you for your comments about the book. I'm not entirely sure how we managed to miss The Netherlands from the list of countries. I will see what we can do about that.

    In the meantime, you can find out the information based on your latitude and longitude from NASA figures here:

Hi Michael,
Having recently purchasd your publication, which I have found extremely helpful, I need clarity regarding the placement of the batteries.
I intend having 3 deep cell batteries, 6 cells each and 165 amps. I would like to place these into my roof.
I am aware of the hydrogen hazard and hence why the question. The roof is pitched and the floor area is 10 x 16 meters, the apex of the roof is 2.1 m high. There is a small amount of air flow through, but no extraction system as such. Is this acceptable or not?
Many thanks,
kind regards,
     Judi Robinson, August 1, 2010

    There will be a small amount of airflow in the roof space to make siting bateries there suitable. However, it is difficult to give accurate advice without seeing your site.

    Roof spaces can often get very hot and this can be a problem for batteries in general, and especially as they vent more at higher temperatures. I would be tempted to fit a small extractor just to be safe.

    You can buy a solar powered extractor fan that is designed to be installed into roof spaces which does not require much power.

having a solar panel put on my motor home to keep my batteries topped up,been offered a crystalline and mono crystalline panel,which in your opinion would be the best .
     alan johnson, August 2, 2010

    There is not a lot of difference to be honest. The physical size of the mono crystalline panel may be slightly smaller but otherwise there will be no real difference in operation.

Do you by any chance have an digital copy of your book that I can load onto my computer for a quick reading reference? I think Amazon may have a digital book, but it is designed for "digital readers" rather than for a straight working computer. We're buying the printed version, but could really use its digital equivalent.
And, you have a great website!
     Tom Meeker, August 3, 2010

    Thank you for your comment about the website.

    We don't have a digital version available at present, but a digital version of the book in all the usual digital book formats (Kindle, ePUB, PDF) will be released this Autumn.

Hello Michael,
i have just ordered your book after going through this website,i found it a spot on resource.we have a requirement to power a billboard with solar pv in Ghana\West Africa
the system will power 5x400 watts halide bulbs for 8 hours and the system will be a stand alone,can i ask if deep cycle batteries can be placed (hidden)under ground to make it less conspicuous.i will really appreciate any advice on insolation, solar panel, and battery type.
many thanks
     James Yasembi, August 4, 2010

    Siting batteries underground can make a lot of sense as they are more insulated against extremes of temperature. The only thing you need to ensure is that they are accessible so you can check and water them regularly and that there is an extraction system in place to allow the batteries to vent properly.

    You can find the insolation figures for Ghana in the 'Online Calculators' on the web site. You've got quite high power requirements, so your system is not going to be a small one, but you also have excellent solar irradiance throughout the year so that will certainly help you.

I have your handbook and have read it several times, I do need some help if you would please. I need help hooking up my battery's ( total of 4 ) ( 2x12 volt & 2x 6 volt ) I will be using a 12 volt system. From left to right with the 12 volt battery's with the 12 volt + up at the top on the left side of the battery bank. First connection would be to parallel the + terminal to the + term of the second 12v battery, same for the neg side ( I have that with no problem ) my problem is connecting the last two battery's both 6 volt to have them parallel with the 12 volt battery's. Then once I get that part done I know the charge controller ( and inverter ) positive side would be connected to the + terminal at the first battery. Where would the Neg - side of the charge controller and inverter go. I know in your book you want us to connect the neg - side at the last terminal to have equal discharge from the battery's, until I know exactly how to wire the last two battery's to the first two I wont know where that is. Last question I promise, I guess its best to go ahead and install a ground rod and tie the Neg - side of the battery's to the ground rod ( Am I reading that correctly ? ) Does it matter what battery you come off of? O.K. Wew! I think that's it, and thanks again for your assistance.
Joe Groves
     joe groves, August 5, 2010

    You'll have some problems with having different types of batteries in the same system, because they will charge and discharge at different rates. You will end up with an imbalanced system.

    My recommendation is that you have a set of identical batteries and ditch the others. Alternatively, you can set them up in two banks of batteries - one bank with all the twelve volt batteries and one bank with the two six volt batteries and switch between them. This would have the benefit of allowing your batteries to rest occasionally which will improve battery life.

    So you connect your 12v batteries in parallel, connected to a battery switch, you connect your 6v batteries in series and connect them to a battery switch, and then connect the battery switch to the controller. You can buy these battery selector switches from most solar suppliers. Here is the sort of thing I have in mind:

    Solar Battery Switch

    With regards to the ground rod, connect it to the negative terminal on the first battery in the series - i.e. between the battery and the controller.

    I hope that helps!

I just flew from Albuquerque NM (my home) to Oregon and along the way, I read your book. First off, well done on making a potentially complex subject seem rather simple. As an electrical engineer I understand more than most so perhaps that helped. In any case, well done. Now my question. Regarding grid-tie, why isn't there a configuration that allows one to use batteries to store from the solar array and then once the batteries are fully charged, offload the additional energy from the array onto the grid? If I read correctly, the grid tie configuration means you are using the energy as it's produced and when not being used, it's directed to the grid. Nothing about storage involved. Is this due to safety concerns or the potential for the grid to drain the batteries? If so, rectifiers could easily fix this. Just curious.

     James Culbertson, August 11, 2010

    Thanks for the comments. With regards to grid tie systems, I've shown a standard grid tie system and a grid-fallback system in the book but you are absolutely right, it would be possible to create a system as you have suggested, incorporating battery storage so that you are always (as far as possible) using your own energy, but putting any surplus onto the grid.

kindly update me about the latest inventions in soler power
     adnan mahmood, August 11, 2010

    What a huge question! There is so much going on with solar power at the moment, it isn't easy to know where to start.

    In a nutshell, solar is improving in three areas: solar thermal, multi-layer photovoltaics and thin film solar paint.


    For large solar power stations, the technology is moving away from solar photovoltaics (although these still play their part) and moving towards solar thermal systems, creating very hot water and powering turbines from steam. Efficiencies are high and the benefit is that the hot water can be stored for hours, allowing the energy to be released when the power is needed.


    Photons are light energy. Released from the sun, these bundles of energy provide us with light and heat. Photons work at different frequencies, including visible light, infra-red and ultra-violet frequencies. Currently, solar panels work on visible light, absorbing these photons and generating electricity. A lot of research is now being done on capturing some of the other light frequences: infra-red and ultra violet aren't visible to human vision, but are not absorbed by clouds in quite the same way as visible light. This means they can be more effective on cloudy days. By capturing more frequencies of light, solar panels can become more efficient. We should start seeing these panels on the market in the next couple of years.


    A lot of work is being done on making amorphous panels cheaper to build. The technology here is to use multiple layers of so called 'solar paint' or 'solar ink' to paint a surface (typically glass or aluminium) to make a very low cost solar panel. Whilst these aren't as efficient as other photovoltaic panels they have the very big advantage of being much easier and cheaper to produce, thereby bringing the price of solar power generation right down. Expect to see these panels incorporated into all sorts of devices such as mobile phones, laptops, MP3 players and the alike in the next couple of years. This technology can also be used to clad new-build houses and factories and the price of it will become so cheap that it is possible that solar roofing materials will be almost as cheap as non-solar roofing materials in the next five years.

    I hope that answers your question.

Dear Michael,
Thanks for a very informative and great value book. I have just bought 3 x 100watt solar panels and a 250 watt grid tie inverter off eBay. The total cost was about £700. I’m planning to install it at a holiday property near Marbella which is occupied about 20% of the time. The insolation figures on your website indicate that Marbella has an average of 4.7 kwh/m2/day. Can I simply multiply this figure by the number of watts my panels generate (300) to get the average output per day (less efficiency losses)? If so, this gives about 1.4 kwh per day, does this sound reasonable or am I missing something?
Also, what is your opinion of the small ‘plug and play’ grid tie inverters available on eBay. I paid £90 for a 250 watt grid tie made by ‘Sun’. I have tested it and it seems to work OK, it produces a pure sine wave output and has island protection.
My plan is that this small system will produce enough power to run the swimming pool pump, fridges etc when the property is unoccupied. Does this sound reasonable to you?
Any help or suggestions would be most welcome. Cheers, Paul

     Paul Newton, August 17, 2010

    Thanks for your comments about the book. Your project sounds very interesting. In terms of the calculation, you are correct - so long as you take into account all the losses in the system.

    With regards to these 'plug and play' grid tie inverters, they are not certified in any country for true grid tie connection as they do not have any of the safety cut out features of the proper systems. However, as a simple way of taking a house off-grid, or to provide 'grid-fallover' power, they can work well.

    It is impossible for me to say for sure whether your system will produce enough power to run a swimming pool pump, fridges and so on without knowing the wattage of the individual pieces. However, a small swimming pool pump and a low energy fridge should be fine with that setup.

    Enjoy your solar power!

This question is regarding the output of your solar irradiance calculator that provide the results in sun hours/day or kWh/m2/day. Have the DC to AC derate factors already been factored into this number?
     Bob Sisco, August 22, 2010

    No they have not. There are a number of derate factors that have to be worked out, depending on the type of system you have, all of which are discussed in the book.

Thank you very much for the chance to ask you a question, on finding this site I have ordered your book. I do have a question that I would like to ask. I have an installation of 12x24 Sanyo 240 panels and an SB3000 inverter. The problem I am experiencing is that in the afternoon I am getting on sunny days a sudden dip in power from 2.5kw down to 500w. I have a very small amount of shading starting at that time (very small 1/20 panel) and the power output will stay at 500w most of the day climbing very slowly. What I have found is that if I reset my inverter the output goes back up, can you explain what is happening.
Many Thanks
Gary Hancock (Coventry)
     Gary Hancock, September 3, 2010

    Thank you for your message.

    I've not come across this particular problem before. You're fault sounds like a problem with the inverter. At a guess, I would suggest that the voltage increase in the afternoon sunshine is causing a fault in the inverter.

    If it is safe, I would suggest that you check the voltage from your solar array in the morning before the fault occurs and then check it again when the fault kicks in.

    I suspect you're going to need to change your inverter.

Dear sir
I have found this link by accident (Google). Now, I am want to know the solar irradiation data for different parts of Nepal, which is not listed here. Is there anyway that you can help me with this issue?
I would appreciate your comment and help.
Thanks a lot.
T. paudel

     Trilochan Paudel, September 3, 2010

    We are planning an update on our irradiance tables at the end of next week. Nepal will be one of the countries we will be adding to the system. (NOTE: this has since been done)

If two light of capacity by 80w replaced by 20w which run for 6hrs per day what will be energy saving per month in kw/hrs?
     vijayraj, September 4, 2010

    If you have two 80w bulbs and are replacing them with 20w bulbs, then you will be saving 40 watts per hour.

    Multiply the 40 watts by 6 hours to work out your daily watt-hour saving (240 watts-hours per day). Multiply this figure by 30 days to work out a monthly energy saving: 7200 watt-hours, or 7.2kWh.

Hello M
My question is about charge controllers, I have used PWM and Mppt along with the old shunts, I have been seeing "CMP" controllers advertised, some of the other controllers the company make do mention PWM and MPPT but this one is only CMP so I was wondering if it was new packaging for the old style shunt..
Another thing I wanted to say, its one thing having a great book, but to back it up with a great website is exceptional! Thanks John Joines Peralta New Mexico USA
     JW Joines, September 6, 2010

    Thanks for your question and for the comments about the website and book.

    As for CMP controllers, you are right, they are shunt controllers. Manufacturers claim they are more efficient than the older shunts and I've used them on small applications such as powering vending machines or providing remote lighting.

    I was recently working on a project to run a vending machine on solar power which had to work even if the solar panel was always in the shade. To my surprise, I found a cheap $15 Chinese CMP controller was the most efficient for my application. Which just goes to show, you can learn something new every day.

I'm in the Philippines and it could take some time before I get your printed book. Do you have a pdf version (I will pay of course by PayPal if possible)?
Best regards,
     Michael, October 8, 2010

    The book is now available on Amazon in Kindle format. Other formats will appear in the new year.

Hi, my question is can a 12V solar panel be use to charge a 6V sealed lead acid battery? Is this safe and will not damage the battery since is 12V Vs 6V ? The sealed acid battery has 3 different charging levels and each levels uses different voltage like 7V or lower.
Thank You.
     Eason, October 14, 2010

    You cannot use a 12v solar panel to charge a 6v battery. There are two options - to charge two 6v batteries in series, or get a voltage converter to drop the maximum voltage of the solar panel to around 8 volts. The latter is a very inefficient solution.

    You can buy 6v solar panels for smaller projects. Unfortunately they are not cheap and it is worth shopping around, but they would be the best solution to your problem.

wonder if you could answer my question, hopefully the solution is simple!
Installing Solar PV in conjunction with Back Up Generator
Completed a quote for PV installation on a farm, problem is there is an Automatic Back Up Generator that kicks in if the Grid fails, concern is: If the Grid fails and the Generator kicks in and is supplying the house with power at say 1kw, the PV is say Generating 4kw where does the other 3kw go?
Will it damage the Generator?
Is there any sort of protection i can put in to stop any damage?
The Grid comes into an outhouse where the Generator is situated, this feeds the farmhouse via a SWA cable to consumer unit in house. I was going to feed the PV from the Inverter into the consumer unit in the farmhouse to avoid running another SWA cable back to the outhouse. Obviously the original SWA cable shares the Grid voltage/Back Up Generator.
Any advice would be much appreciated

     Jim Ross, October 14, 2010

    I suggest your best solution would be to use an intelligent power inverter that can manage both solar input and generator. These work by using solar where this is available and switching to generator input if there is not enough power being generated by the solar array.

    This can then stop and start the generator where required and manage the level of power input as needed.

    Having one unit that manages both power input ensures that the AC waveform is synchronised and stops the two systems from 'fighting' each other and reducing the efficiency of each.

    I believe Outback Power do a suitable controller/inverter that will do what you need.

    The other alternative is to use a transfer box so that the solar and the generator work independently of each other. However, that is a very crude solution that won't allow the generator and the solar array to work at the same time - not such a good solution.

Hi, Just got your book in the post, v interesting, what I would like to know is Has it been translated into Spanish and if so, where can I get hold of a copy for my husband to read?
Look forward to hearing from you,
     Liz Rosales, October 22, 2010

    Sorry - there is no Spanish version of the book at present.

how i can sizing my solar panels for my total load 30 kw day (ac )
     hussein mohamed sherif, October 24, 2010

    Using the solar irradiance calculator, select your nearest town to identify the average daily insolation figures for each month. Take the lowest figure across the year and divide your load - 30kwh - by this figure. This will tell you in rough figures how large a solar array you will need.

    This does not take everything into account, such as losses and angles and so forth, but will give you a reasonably accurate figure to start with.

Where are get solar book in Bangladesh.
     SUMON, October 29, 2010

    I do not know of any book shops in Bangladesh who stock the book. However, it is available from Sita Books in India ( or from The Book Depository ( who will ship it to you free of charge.

    Sita Books usually hold our books in stock and can dispatch one to you very quickly.

On-line calculator for panel array angle does not allow me to type in the name of the community. I have the book and instructions but can't access that requirement. Thank you
     Jacques Bakke, October 29, 2010

    Instead of typing in the name of the community, you will need to select the name of your nearest town from the list (press the down arrow on the right hand side of the data entry box to get a full list).

    If there is no town listed that is close to you, let me know where you live and I will get the information for you.

Dear Mike
i life in Angola Luanda , i have business witch install and sell solar system
i got your book online , was very helpful , i wish i had it one year a go .
i trying to find the Solar Irradiance figures for Angola . unfortunately on your site you dont have it . how can i get it ?

     fadi nassar, October 30, 2010

    Thank you for your comments about the book. I will update the site to include the largest 10 cities in Angola later on today.

I just ordered your book. I have been excited about getting my hands on some different ways to generate energy for my house and my community. With that being said, do you have other materials that you would suggest on reading on creating a "solar farm"? I have some large chunks of land that are passed down to me that used to be tobacco fields but now are nothing except for weeds. I was just wondering how to go to something that can help power the "grid" (which I am trying to learn about) for the local community. Just wondering your thoughts. Thanks, Ashley
     Mr. Ashley Waldridge, November 1, 2010

    Wow. That sounds like you have a big project there. I would recommend that you talk to your local electricity company and find out what they are looking for from electricity producers. I would also suggest you talk to your local government office as they may well have some great contacts (and possibly even help with the funding) for your project.

I have a big challenge. I'm currently drafting a proposal for my boss on powering Telecommunication Base Stations using solar power and at the moment can't come up with relevant information to aid the determination of number of panels, inverters, batteries and control chargers required.
Power consumption for base stations being considered are 3-4KW and 8-9KW.
Average sun peak hours is 4hrs.
Thank you for your time. I look forward to hearing form you.
     kunle, November 1, 2010

    Average sun peak hours does not give me enough information to work from, as the levels of irradiance will change throughout the year. You also do not say whether the power consumption is measured as an hourly figure or a daily figure.

    First, you need to identify the minimum sun levels at any period in the year - assuming you want your system to work throughout the year.

    For the purpose of my answer, I'm going to assume that you have a minimum of 2 hours peak sunlight (total daily solar irradiance) a day and that the power consumption is 4kWh per day.

    Divide your power consumption by the irradiance figure to work out a rough size for your array. In this case, divide 4kWh by 2 = 2kW array. Then add 20% to take into account any losses in the system - 2.4kW array. This is the size of your solar array.

    You need enough batteries to provide a few days of hold-over, so if the weather is worse than normal for a few days, your system does not run out of energy. I would suggest a five day hold-over for your application. If your system uses 4kWh of energy per day, multiply that by five and then add 20% - 4kWh x 5 = 20kWh * 1.2 = 24kWh. This is the size of your battery pack.

    You can now size your solar controller based on your average and peak power consumption. I would recommend a MPPT controller to get the maximum power out of your system.

    In terms of budget, take the size of your array (2.4kW) and multiply it by $3,500 to come up with a rough rule of thumb price. (2.4kW x $3,500 = $8,400 per installation).

would it be possible for you to add St.Vincent & the Grenadines to your solar calculator. Your book was very useful. living on a tropical island I can make the most of the sunshine. Thank you.
     Nicky, November 2, 2010

    I will add this information to the web site later today.

Does the inverter make a constant humming noise? If so, how loud is this for a small system (10 kW)? How loud would this be for a large "solar farm" system? I'm sure it varies, but I would just like a general idea, so I can assess whether this would be a nuisance or not. Thanks.
     Debbie, November 4, 2010

    Some older inverters hum, but the modern ones are silent.

    For a huge solar farm, you either have multiple small inverters or one huge one which would be installed in a small building on the site. I've seen some large wind farm inverters in the past and yes they make a hum, but the noise is completely shielded by the building they are fitted into: from outside you can't hear anything.

    Inverter noise certainly should not be a nuisance.

I just used your solar angle calculator, and it states on the final calculation display; "Figures shown in degrees FROM? vertical" should this read figures from horizontal?
     mike, November 5, 2010

    No - the figures should be from vertical, not horizontal.

    The solar array should be almost vertical in the depths of winter when the sun is low in the sky and almost horizontal in the height of summer.

    The confusion comes from most solar calculators on the web showing the figures from horizontal rather than vertical. We chose to do it the other way around because most solar panels are either mounted on a wall or roof of a building and working from vertical is often an easier way to carry out the calculation.

On you kindly show solar angles by month - thank you - just what I needed. For the sake of completeness I would like to know when in a month i.e. the first, 15th, 21st or 31st? for instance, these angles are valid. Having the values centred on the 21st (equinoxes) would make most sense to me, but hey, what do I know!
     David, November 11, 2010

    The figures are taken from three hourly samples throughout the month, based on an average over 22 years.

    The figures take into account average hours of day light and average cloud cover over the period of the entire month.

Hi, I have a sliding glass door that is 6'-6" tall and the sunlight is at an angle of 25.5 degrees above horizon, how far into the room will the sun strike the floor?
Please show math
     John, November 12, 2010

    Please let me know the name of your maths teacher and I'll submit the homework under my own name.

hello michael,
i read you handbook this weekend and wanted to ask some questions, as your example only addresses a straightforward 12V system (12 array - 12 volt battery bank, 12 volt DC in the cabin).
my setup is a bit different, let me explain:
i am looking at setting up an off-grid solar system for my island house in the south of thailand, and would like to ask for some help from you:
given my peculiar site location i have very long distances to cover, the longest distance between my PV array and the most removed light is 50 meters (150 feet). inside the house, the cables are inside the walls, and therefore not easily accessible, in other words, i cannot change the wiring. the cables inside the walls are standard 2 square milimeter diameter copper cable. i have dome some calculations according to your example and formula and it is clear that i cannot run a low voltage DC current with these distances and the cables already installed. i need a 220 V system inside the house.
my daily requirement is 1kWh (1000 Wh), that already includes inefficiencies of both the inverter and the batteries. based on my calculations i would need (depending on whether i set up a 12 V, 24 V, or 48 V system) the following battery capacities:
12 V - 5000 Wh/12 V = 417 Ah
24 V - 5000 Wh / 24 V = 209 Ah
48 V - 5000 Wh / 48 V = 105 Ah
the distance between the PV array and the battery bank can be as short as 5 meters. for this part of the system i can and will of course use a thicker cable.
i would like to have your expert opinion and advice on which voltage i should choose for my system? i am thinking 24 V would be good, with 4 panels @ 100 Watt each (i do have for identical 100W panels, they are rated at 17V nominal voltage, 5.89 Amps), and 4 batteries of 150 Ah each (2 in parallel, then in series to create my 300 Ah @ 24 V battery bank).
or should i run it at 12 V with 4 panels in parallel and 4 batteries in parallel? that is the only part that did not become clear from reading your book. what are the pros and cons??
thank you very much, i look forward to hearing back soon.
best wishes
     julien, November 14, 2010

    Thank you for your question. It sounds like you have an interesting project.

    Looking at what you are saying, I would agree that you need to go to a 220v system inside the house.

    The benefits of the higher voltage is simply that you can use thinner cables. Because you are going to be running an inverter, you do not need to have any particularly long cable runs at low voltages - unless the inverter is going to be some way from the battery bank.

    If the battery bank and the inverter are close together, I would recommend 12v or 24v, simply because you then have a larger choice of inverters at a lower cost. If your battery bank is going to be some way from your inverter (i.e. over 20 metres), then you may well be better going for 48 volts, simply because your losses will be lower and the system will be easier to install.

dear michael, i just tried to reply to your email from a couple of minutes ago, but it bounces! so here i am again...
the distance between PV array and batteries is of 15 feet, the distance inverter is only 8 feet from the battery pack. so, short distances!
i still don't know which configuration is better, 12 V or 24 V? i don't think they are identical, right?
if i have 4 panels (100W each, 17 volts nominal voltage) and 4 batteries of 150 Ah each, which system will give me more/better supply for the 220 V system?
option 1:
wire 2 panels in parallel and then the 2 strings in series (thus creating 24 V) and to do the same with the batteries
option 2:
wire 4 panels in series (thus creating 12 V) and to do the same with the batteries
what is the fundamental difference (other than the cable thickness, which is not an issue because of short distance, plus i will use a big cable anyhow to avoid the risk of even the slightest inefficiency) between the 2 options? would a 24 V inverter be more economic than a 12 V inverter to 220 V?
another question that i have is:
since the panels are not 12 V, but 17 V, i will in actual fact create a 17 V or 34 V system. the batteries however are 12 V, is that good or bad? my thinking is that it puts extra stress on the charge controller to even out these differences, isn't it? would there be another way?
many thanks for your help michael, much appreciate it.
     julien, November 14, 2010

    For your application there is no real difference between a 12v system or a 24v system in terms of efficiency. With the information you have given me, my personal preference would be for a 12 volt system, because the components themselves tend to be very slightly cheaper, and also if you need to get hold of replacement equipment at some point in the future, you can normally get hold of 12v equipment quicker.

    With regards to the solar panel voltages, you require a higher voltage in order to charge the batteries in the first place - i.e. you cannot charge a 12v battery from a 12v source, you need 14+ volts in order to charge a 12v battery. The charge controllers are designed to even out the higher voltages and if you go for an MPPT controller, they will ensure additional current goes into the batteries when the solar array is putting out its very highest power.

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