Solar Solutions - Ask Me A Question

Michael Boxwell Do you have a question about a solar solution? Do you want to get in touch? Have you got any questions about solar, or about the book? Do you have a particular problem with a solar project that you would like an answer to? Have you found a mistake in the book? Have you got any suggestions as to what you'd like to see in future editions of the Solar Electricity Handbook? Whatever it is, I'd like to hear from you.

You can contact me through the contact form below. Or you can browse the questions that other people have asked and read my answers below.

Get in touch:

    Your name:

    Your e-mail address (this will be kept private but will allow me to get in touch with you to give you an answer):

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Previous Solar Questions and Answers

On the online solar calculator the pull-down menu for panel direction only caters for the northern hemisphere. There is no option to face the panel north. Or have I missed something obvious?
     Geoff Smith, November 9, 2014

    That is being fixed at the moment. The new version of the software will work with both north and south hemispheres.

Dear Sir, I have 10 BP solar panels of 50W and 17V each can I, using an inverter power a immersion heater which has an input of 1850w at 220v A/c, I live in southern Spain I would use this independantly of course.
Thank you
     Peter Bennetts, November 12, 2014

    Yes, you can, but it does depend on how much you need to use your immersion heater. Your 500Wp solar array would generate around 900 watt-hours of energy on an average day in December, which would allow you to run your immersion heater for half an hour. In March and September, your solar array would generate around 2.1kWh, which would allow you to run your immersion heater for a little over one hour, whilst in the middle of summer, your solar array would generate around 3½kWh, which would allow you to run your immersion heater for around two hours a day.

    Because of the drain of your immersion heater, you are going to need a fairly substantial battery pack and inverter, neither of which is going to be cheap. You would find it far more cost effective to use a solar hot water system to heat your water.

i want to know insolation on particular place in hourly basis means (6am-7am) what is the insolation in kw/m^2.
how to convert kw/m^2/day into kw/m^2 can solar operates above 1 kw/m^2
     dipak, November 13, 2014

    Unfortunately, our systems cannot provide hourly calculations at present. You can get that information from NASA, but be prepared to do a lot of calculations and extrapolations yourself:

    KW/m^2 is a power rating whilst kW/m^2/day is an energy rating. You cannot simply convert one to another, but the NASA website will also provide you with the power rating if you wish to use it.

    Solar itself can operate above 1kW/m^2, the limitation is how strong the sunlight is.

wanted to ask if northern areas of PAKISTAN are recommended for solar power generation solution? also wanted to calculate the angels for the northern areas which i couldnt find in the option list...can you kindly help
     Kashif Khan, November 13, 2014

    You can run solar in Northern Pakistan. In fact it is a very good area to use them because of the amount of sunlight you get throughout the year.

    With regards to the calculators, use the settings for Islamabad.

Hi Micheal;
I am a PhD student in the UK and I am looking to find out if a camera that capture the settlment of DUST on PV panels does exists and where I can purchase it?
I have purchased your books twice for two different years, can you please tell me which is the best book which explain 'Solar Irradiance' beside you book?
Thank you
     MR Mourad Elbouchikhi, November 13, 2014

    Thank you for getting in touch and for buying the book. I'm not sure that there is a camera that will do what you need. However, the technology does exist because it is used by car manufacturers for automatic windscreen wiper sensor systems.

    That could be an interesting piece of technology to develop, monitoring how much dust and grime there is on a solar panel and then alerting the owner when the panels need to be cleaned.

    If you really want to understand solar irradiance in detail, go and look at the NASA website. The Atmospheric Science department has a huge amount of information. Start here:

I am interested in building a grid fallback system. Your book is an excellent reference manual except that it does not appear to cover Australian regulations in relation to grid disconnect. Australian safety regulations require automatic grid disconnect in the event of mains failure so as to automatically remove dangerous voltages form the grid system. The diagram of the grid fallback system on page 189 indicates a manual AC isolator switch, which would not be acceptable. Can you elaborate, with wiring diagram, on an automatic isolator.
     Ken, November 14, 2014

    With a grid fallback system, power is never provided from the solar array to the grid. Instead, the solar energy system charges up local batteries which is used to provide the power for the building. When the batteries are exhausted, the system then switches to the grid to continue to provide power for the house.

    As a consequence, there cannot be any power going from the solar array back into the grid.

I am researching solar thermal for domestic water and space heat in the Halifax Nova Scotia area. My question is would the solar irradiance table on this site be relevant for evacuated tube panels (factoring in efficiency and effective area) ?
Any additional info or literature that you can recommend would be appreciated.
Thanks in advance.
     Darel, November 14, 2014

    Absolutely. The calculations are the same.

    I'm not an expert in evacuated tube panels so I cannot advise you particularly on this area.

I am running a 3kw system and I am getting 82v out of the solar panels which is correct, but when we hook up the wires to the solar control system, the voltage drops down to 49v when tested with the voltage meter. Do you why this would be?
     Mike edmonds, November 18, 2014

    Are you measuring the open voltage of the panels when you get 82 volts? If so, this would be what I would expect. As soon as you put a load on the panels, the voltage drops to around half the open voltage.

we have a 240 volt spa pool. Can we get panels that will produce enough energy to run spa pool? Do we need batteries t store the energy?
     Brenda, November 25, 2014

    Yes, you could run it from solar, but it is going to take a lot of power. Find out the wattage of your spa pool and then use the Off Grid solar calculator on this web site ( in order to work out exactly what you need.

Dear Michael,
I'm really interested in the free report, although unfortunately I got some kind of error and got a lot of script rather than an attachement. I've tried twice. Could you please check if I can get the correct one? Sorry for the inconvinience, thank you for your help,
     Gaston, November 25, 2014

    Hello Gaston, Thank you for your e-mail.

    The report is sent in HTML format, which means you need to enable HTML in your e-mail reader. If you cannot do that, then save the report on your PC and open it in your web browser.

sir , tell me the correct angle and correct direction of the solar pannel in my city muzaffar nagar (uttar pradesh) in india .kindly sent the angle and direction of the pannel
ashwani kumar
muzaffar nagar(u.p)
india pin-251001
     Ashwani kumar, November 26, 2014

    The ideal direction is South. The angles will depend on when you wish to optimise the power generation.

    The best angle for all-year-round generation is 60 degrees. The best angle for winter power generation is 36 degrees. The best angle for optimum summer generation is 84 degrees.

    I hope this helps.

I want to purchase your Solar Electricity Handbook 2014 edition in an ebook format. Thats why because I am living in Turkey and it is a pain to import the hardcopy to Turkey.
As I saw there is an ebook version of the book. But I could not find where to purchase.
Can you help me?
Kind Regards,
     Cenk YÜZEREROGLu, November 27, 2014

    Thank you for your feedback. The book is available in Kindle format and can be purchased from The link for the ebook edition is

Hi Sir,
hope you are good and dooing great!
Actually myself sulabh maithil from india, and dealing in solar street lights,
recently have installed -30 watt, LED high mask street light in an town (muradabad,india), specifications are below..
30 watt*4 - LED lights
200 whp*2 solar panel
100 AH*2- battery,
6 mm hevells wire.
6 meter pole.
autonomy-12 hrs
as per my calcution i did all things rights and installed them at the palced asked for.
Problem- lights gets on at right time in evening by the sun sets but stop in night by 2 am(midnight),while it supposed to stop at @ 6 am. sceen is like ,since on one pole there are 4 Led lights of 30 watts each,stop lighting one by one.
please let me know what should i do and what needs to check to get the light till 6 am i mean 12 hrs light.
looking for your precious sugeestion..
Thanks in advance....
Sulabh maithil.

     sulabh, November 29, 2014

    Thank you for your e-mail.

    Does your solar controller have a display that shows how much energy is being provided by the solar panels? Excluding inefficiencies, your solar array should be generating between 1400wh (December) and 2380wh (May) of energy each day, +/- 20%. If we take into account the inefficiencies of the batteries, that probably rounds down to 1250wh - 2400wh of charge per day. If it is below these figures, then check for shading issues and check the resistance of the cables and connectors between your array and your controller. Check also that the amount of energy you are getting from both panels is comparable. If one is significantly different from the other, you may have a faulty panel.

    If your lighting is coming on at 6 o'clock in the evening and needs to run for 12 hours, you are going to be using 1440wh of electricity during that time. If we factor in some inefficiencies into the system, we can round that up to 1500wh.

    Your battery storage is a maximum of 2400wh under perfect conditions, which means you are discharging them by at least 60% each night assuming a 12 hour usage. That is quite high and will reduce the lifespan of your system.

    You are also providing quite a big charge for your batteries in a relatively short period of time during the day, and this is not ideal for such a small battery pack.

    You need to double the size of your battery pack. Assuming the solar array itself is working fine your system should be generating plenty of power, but because you are pumping so much energy into the batteries over a relatively short period of time and then draining the batteries to around 40% state of charge each night, I suspect your batteries are struggling.

    Add two more batteries to your battery bank.

Hi. Are you aware your free reports are delivered including all the programming code within the document and are not an attachment? Thanks for all your work. Love your book.
     Steve Savina, November 29, 2014

    Dear Steve

    Thanks for the comments about the book. The reports are presented in HTML format, which formats correctly in most email programmes. If you do not have HTML enabled, however, the formatting is incorrect. To resolve this problem, save the attachments and open them in a web browser.

I see manufactured panels with 290 w. I don't see where I can even build a similar panel, especially for a little over 200 $$
     Kevin, December 1, 2014

    Please do not try making your own solar panels. Solar panels are very intricate pieces of technology and most home-made solar panels that I have seen are downright dangerous. The heat build up in any panel is immense and many home-made panels are a fire hazard.

    You can buy a professionally manufactured 200Wp solar panel on for around $240 and a 100Wp solar panel for half of that. These will provide far better performance, far better reliability and have a much longer life than a home-made panel.

    For more information about why I do not recommend making your own panels, read this article:

    All the best

Hi Micheal,
Greetings from Malaysia!
I find your Handbook relevant and interesting.
I intend to fix a 4kW, 16 panels Canadian solar with SMA inverter.
My house location latitude is 5 degrees 22 minutes North. Both the front and back roof have a tilt of 15 degrees facing NW and SE respectively.
Please give your recommendations and comments on these:
1. Where should I place the panels, all 16 pieces facing the back or front? Or a combination of both directions?
2. I intend to put a water sprinkler system that runs for several minutes (at intervals of maybe few hours) for cooling purpose. This idea is to optimise on higher output efficiency due to temperature coefficient issue of -0.42% to -0.45% for Canadian Solar panels. FYI, water is aplenty & cheap here.
Looking forward to your respond with much appreciation.
Thank you very much in advance.

     Loh Chee Yoon, December 2, 2014

    Thank you Loh.

    1. Place the panels at the back.

    2. I'm not convinced that a water sprinkler system is going to be needed in Canada, although I could see that being useful in California. It is an interesting idea that could also help to keep the panels clean, but my gut feel says that any power production benefit is going to be minimal.

    What I would recommend is that you fit the panels and see how you get on. Mount the panels so there is an air-gap between the panels and your roof and you will probably find that this is sufficient to keep the panels at a reasonable temperature.

Sir!Can I run 24v 0.5hp,16amps motor directly on 12v 150watts solar panel? otherwise any ac to dc 12v adaptors to run motor?.
     kiran, December 3, 2014

    If your motor is a brushed DC motor then yes, you could run it directly from the panel, but it would not run particularly efficiently.

    The motor speed would vary depending on how much electricity the panel is generating at that moment in time, which can vary dramatically, particularly on a sunny day when there are lots of clouds in the sky! As soon as the sun goes behind the cloud, the motor performance would drop dramatically.

    Your maximum performance from the motor would be probably be around 1/10th HP.

Hello, when will your 2015 edition be published?
Keep up the good work.

     Joe Choat, December 3, 2014

    Thank you for your comments. The 2015 edition of the book is now available for pre-order and will be available in January.

The to the bookstores presents the 2013 edition.
Will the 2014 edition be available soon?
     Bill Nickels, December 5, 2014

    The 2014 edition is out, but some of the book sellers seem to be linking back to the older edition. I am working with several of them now to resolve the problem. The 2014 edition is available and the 2015 version will be out in January.

firstly thanks for your book
secindly,i want to ask you
iam from egypt in nubariya city in behira near from abo matamer city
i didnt understand how i gt best location for direct solar panels to sun
please reply me
     ismail gamal, December 7, 2014

    Thank you for your comments about the book. I've looked up your location.

    Your solar panels should be south facing. For best all-year-round performance, the panels should be angled at 59 degrees. For best summer performance, the panels should be angled at 82 degrees. For best winter performance, the panels should be angled at 36 degrees.

    I hope this helps.

Hi Michael. i want to generate a small amount of power to illuminate a work area and can get away with using 4 of these :
Lenses Type : Crystal Clear
Case Style : 5mm Round Straw Hat (Low Profile Dome)
Viewing Angle : 170 degrees
Brightness : 13000mcd (Ultra Bright)
Forward Voltage : 3.2v - 3.8v
Forward Current : 20mA (typical), 30mA (max)
The problem is I would only be using artificial light and the light from a welding arc to generate the electric through the solar panel. Would this be possible? The panel would be a small flexible one rated at 3.6v, 50mA and .18w. I don't want to use a battery but wire directly to the lights if possible.
For your information I brought the 2009 edition and thought it was brilliant and fully intended use it for some projects, unfortunately as is often the way life got in the way and I have just come back to it. I only want to do small projects such as lighting garden paths and use low voltage led lighting, where mains electric would be impractical. Do you know If you have to be registered to install low voltage systems such as path lights, in the UK?
Many thanks

     Martin Downes, December 8, 2014

    Hello Martin, thanks for the comments about the book.

    If you are doing low voltage, stand alone systems here in the UK, you do not have to be registered in any way.

    If you are not going to use batteries, you are going to need a bigger solar panel. If you are going to be using four lights (I presume these are LEDs) then you'll need to budget for 130mA. Your solar panel will peak at 50mA, but most of the time will be far less efficient than that, particularly at this time of year. You'll also have the lights dim on you when the sun goes behind a cloud as well as at the end of the day.

    Personally, I'd use a couple of rechargeable NiMH batteries to smooth out the peaks and troughs and use a 1w solar panel. Instead of going to the expense of a separate charge controller and battery module, go and have a look on eBay for a 'AA SOLAR BATTERY CHARGER'. I bought one of these a few years ago as part of a similar project that I had and it worked a treat:

Can I download the full insolation database onto my computer for use in modeling?
     Steve Smith, December 9, 2014

    Not yet, but we will be offering this to clients early next year at a reasonable price.

Four 12v panels installed, 100w each at 5.5 amps ea, connected to a charge controller that came with the panels, connected to Four 12v Solar AGM batteries at 125 amp/hr each. I am not using an inverter, but running about "fifteen dc bulbs" to various rooms in the house, connected to the "lOAD" connection of the charge controller. When some of the bulbs are on, if I turn on additional bulbs it would cause one or two of the already lit bulbs to go out and then comes back on on its own about about 30 secs, and them cycles through some of the others. What causes the bulbs to cycle off and on?
     Carlton, December 10, 2014

    How bizarre! It sounds like a loose connection somewhere in your system.

    Have you earthed your system? If not, fit an earth/ground rod and connect it to the negative lead on your battery bank and to the negative or earth of your charge controller.

Hi, I live in Randburg, South Africa. (About 20km north of Johannesburg) This city not listed in your calculator table list to determine monthly solar irradiances. Then, as an alternative, when I select Johannesburg, I have a choice of compass direction to place my solar panel on my roof. My house is north facing, however, you do not give an option to choose north in your table. Any suggestions?
     Dewald Ras, December 10, 2014

    My apologies, our system isn't working properly for the Southern Hemisphere. We are working on resolving this problem right now and should have a working solution in a few days time.

I am looking at installing a stand-alone/off-grid garden lighting system, but would like to include a 12V operated timer switch, to auto switch and control light usage (between 19h00 and 24h00 daily). I am battling to find a supplier here in Randburg, South Africa for such a timer switch. Can you advise a supplier?
     Dewald Ras, December 10, 2014

    Timeguard do a seven-day industrial time controller module that will work for you, although you would need to incorporate a 12V switching relay into the circuit in order to work for you.

    They are distributed in South Africa by RS Components. Details here:

Dear Sir,
How can i guess that, in a particular area how much capacity of solar power system i can install(without autocad software). Is there any calculation, please explain me.
For example consider an area of 1400 sq. m, & 250W module having a dimension of 1639*982*35.
i will be waiting for your answer.
Thanking you
     Vinod Biradar, December 11, 2014

    As a rough rule of thumb, you will get around 135-140Wp per square meter from most solar arrays, by the time you have taken into account the gaps between panels and so on.

I want to make a mini monocryteline soler panel at home

     pranshu bangar, December 12, 2014

    No! Don't do it! Read this article first:

Well how are you, I did quite a lot of research before I bought the book. I enjoyed the book and I now use it as a reference. Very handy thanks. my question is:-
I have a 12v 100w panel feeding a 312Ah battery. This runs a 1/2 dozen lights and a stereo and a water pressure pump.
I have a 24v 180w panel dedicated to a 24v submersible pump. This pump feeds a 1000l tank.
I want to be able to charge the battery with the 24v Panel when not being used for the pump.
my plan:- get a 24v to 12v converter and simply connect the 24v panel through a separate charge controller to the battery. ie. 2 charge controllers to the one battery.
When the submersible pump pulls power from the 24v Panel none or little power will go to the battery but when not being used by the submersible pump all power will go to charging the battery. What's your opinion?
     Tiarnán, December 12, 2014

    Thank you for the comments about the book.

    Buy a 12v/24v programmable charge controller. This will take a 24v solar panel and charge a 12v battery with it. I'd recommend getting an MPPT controller as otherwise you'll lose half the wattage from your 24v panel.

live in michigan. i have 3 100 watt
panels and i have 3 12 volt 35 ah solar battery ad for the last 6 days little to no sun light to charge the battery. system died but lucky the sun came out the next day and charged the battery about 3/4 of the way but if this happens again how can i charge the battery with out unhooking the system are there something i can add to change from solar to ac to charge i would like to know how to get my pc to run on solar i know you have to replace the power supply from ac to dc but no one out the knows how to do it but know i found some one that knows his solar electricity. THANK YOU VERY MUCH BILL JOHNSON
     bill johnson, December 12, 2014

    Hello Bill

    You can buy a solar controller that includes a backup power option, so that if the batteries run flat power is switched through from another source.

    Are you running your system at 12 volts? If so, there is nothing stopping you plugging a battery charger right into your battery pack and charging them up that way. However, you shouldn't leave the battery charger plugged into the system when you are not using it.

    With regards to running your PC directly from a battery bank, you are right, you need to replace the power supply module. It's actually quite easy to do and there are lots of suppliers because there is quite a demand for in-car PCs and PCs on boats and in aircraft. If you do a search on Google for 'IN CAR PC POWER SUPPLY' and you'll exactly what you are looking for.

I have power solar meter device, I started to measure in my region solar radiation in W/m2 . in December time between SR & SS is around 10 hours and in June around 14 hours .
Q1 : for measurement average amount of W/m2 by device for one day ; which time is OK to consideration sunrise , noon , sunset or each hour then get average ?
Q2 : to calculate wh/m2/day for instant in December , if I get as average 150 w/m2 then I have to multiple 150w/m2 x 10 hour = 1500 wh/m2/day = 1.5 PSH , am I right?
     hoshyar, December 15, 2014

    Q1: It depends on how accurate you want to be. You will get far more accurate readings if you do measure it each hour. Otherwise, I would recommend three hourly intervals, making sure that midday is one of the intervals (i.e. in winter, 9am, 12 o'clock and 3pm).

    Q2: Yes, you are correct.

How many (approx) 250 watt panels would I need to power my home that has an average usage of 60 kilowatt hours per day in the Beaufort N.C area ? 28577 zip code. No trees blocking sun. Im thinking just a grid tie system. Just want to get about a zero cost electric bill. Can you recommend a package and cost, I am a builder so can in all myself probably. Thanks Larry
     Larry, December 15, 2014

    Hello Larry, thank you for your message. Obviously solar panels generate more power in the summer than they do in the winter, but if we assume we want a net zero usage over a period of a year, you'll be looking at fifty-eight 250Wp solar panels for your home.

    For seven months of the year, you would be generating more electricity than you are using.

Hi! I havedone several on and off-grid projects with a company in the US in the past but have now started my own company in Guatemala. There is one question I am not sure of. I have a 2kw inverter/mppt charger/ac relay combi and 4 295w trina panels connecting to 4 350 ah deep cycle batteries. The customer has a 47 watt panel already existing. I am running the system at 48 volts to compensate for the long cable runs. I plan to run the panels in a series at 48 volts. If I buy a seperate charge controller for the extra panel can I wire it into the inverter with the 48v series? There is only 1 PV input in the inverter. Can I connect the controller to the panel and combine them both in a combiner box before sending them to the inverter? Is it possible at all to use the 47 watt panel with my set-up? Thank you
     Kyle, December 16, 2014

    You need to check with the manufacturers of the inverter. Some inverters will allow you to connect multiple strings to one input but most of them have separate inputs in order to do this. Check with the manufacturer and see what they say.

Greetings Michael, will we be seeing a 2015 edition of your solar energy book in the near future? I noticed on Amazon that your previous editions were released during the month of December. I'm new to solar and wish to learn starting from the basics to the latest developments.
Jeff Yurkanin
Chicago, IL

     Jeff Yurkanin, December 16, 2014

    Thank you for your message Jeff. If you check Amazon today you'll find a brand shiny new 2015 edition now available.

I clicked the link for the 2015 edition of your book under links and came up with book not found. I also tried last night on other links and your book did not come up. One link had 2013 edition for purchasing in Australia.
Can you recommend a place to buy the ebook or hard copy for australia.
Thanks Paul
     Paul Pascall, December 17, 2014

    Hello Paul, You probably went onto the site as we were changing it! The 2015 edition actually is released on 1st January, 2015. You can buy the Kindle edition from now and the paperback edition will be available in the next two weeks.

Howcome your solar irradiance calculator asks me for the direction that the panels are facing and then gives me the irradiation numbers for a horizontal surface ?
It doesn't matter what direction the panels are facing if it always gives irradiation on a horizontal surface. You (correctly) always report the same horizontal irradiation regardless of panel direction.
I don't see anywhere that I can input the panel angle in order to get my real irradiation numbers. I think you ought to either add a panel angle to the calculation or take out the question about panel direction, as the numbers reported by the calculator in its present state can be quite confusing and misleading.
What am I missing here ?
     rob kleinschmidt, December 17, 2014

    When you go onto the irradiance calculator and enter the information, you can adjust the angles by scrolling down the page and clicking on the icons for 'vertical surface', 'optimal year round', 'adjusted through the year' and so on.

    I'm in the process of updating the website at the moment. I'll pass your comments onto the programmer and see if we can come up with a more user-friendly way of presenting the information.

Can we make use of fiber optic cable instead of copper cables in solar system for homes projects in Pakistan. To reduce use of heavy copper cables.
     Saif Ullah Sanai, December 20, 2014

    Fibre-optic cable transmits light signals and not electricity. So unfortunately, you cannot use them to connect up a solar array.

Does this book also provides the calculation to determine the correct charge controller?
     M. Shabbir Hussain, December 21, 2014

    Yes it does

I am involved in a project installing a solar panel/batt. power system at a small hospital in Mamou, Guinea W.A. My question is concerning a nearby tree which is approx. 50ft. tall and 50ft. due West of the planned location of the panels. Do we have a problem? Must any obstructions that may cause a shadow anytime during the day or month of the year be avoided?
     George Weertz, December 21, 2014

    Because the tree is due West, you will not have a problem as the shading will only occur at the very end of the day when solar generation is low anyway. As a precaution, I would suggest expanding the size of your solar array by 3-5%.

I am installing a solar roof top mounting, and I want to use ducting for electrical cables, which material should I use for these cables keeping in mind that the ducts will be exposed to sunlight, and I need to keep the temperature low for electrical cables. The duct should be long lasting and tough. Waiting for your kind reply.....
     Waqas Ali Khan, December 22, 2014

    Solar cable is designed for outdoor use and are weather resistant and UV resistant, which means it is pretty well protected against the elements. So long as you are using an appropriate weather-protected ducting, this will be suitable for your system.

I loved your book. I installed a hooked t grid system 4 years ago. I would love to go off the grid. Can this be easily converted? How do you see battery technology evolving (Elan Musk's gigafactory)? And is there a way to hook my Tesla battery to give back up power to my house
     mark asplund, December 22, 2014

    Thanks for your comments. Yes, you can go off grid. You'll need to replace your inverter and add a battery pack and charge controller and depending on how your array is wired, you may need to rewire the interconnects.

    Battery technology is moving forward leaps and bounds. This past year I have been working on an electric vehicle battery that is around 30% lighter and 30% cheaper than existing batteries, with the focus on volume manufacturing, maintainability and end-of-life reuse. Tesla are going a slightly different way about it, by focusing on the software and the control aspects of the battery technology, which allows them to use cheaper cells. The Gigafactory is very interesting because they have streamlined the production processes in very clever ways.

    I've not had a chance to play with a Tesla Model S in order to see how the battery pack could be reused, but I have had a look at the Tesla Roadster battery. The simple answer, unfortunately, is no. It can be done, but you'd have to strip down and reassemble the battery pack specifically to work on your solar array, which would then negate its use as an electric car battery.

    I have used an electric car to power back up circuits in a house (so called 'Vehicle-to-Grid' technology) as part of a research project I was involved with back in 2008. I was using a prototype electric car, testing technology that ultimately has made its way into the Chevrolet Spark EV. This provided up to 8kWh of electricity over a 6-48 hour period. Whilst technically the test was a success, it did raise a number of battery longevity issues, not all of which have been resolved.

Hi, I have a cabin on an island 60km north og Bergen i Norway ( 60°43'29.8"N 5°0'45.5"E) The main source for energy is solar panels....
I am looking for a html code that can give med the daylight time ( today= 5 hours, 44 minutes).. Do you now if anybody have made such a piece of code?
(Your book looks interesting... I willl order one tonight )
Merry Christmas
     john frode, December 22, 2014

    Hello John, thanks for your message. If you simply want to look up daylight time from a website, there are a few of them around. This one looks good:

    However, I'm not aware of any code that gives the amount of daylight time that you can then embed into your own website or application. Calculating the exact figure is actually quite difficult because the earth isn't a pure sphere and the orbit around the sun is an ellipse rather than a circle. However, I found a comparatively simple calculation that is supposedly accurate to within two minutes (which is probably accurate enough!) on this website: Scroll down around half way down the page and you'll see the calculations.

    I have not tried this myself, but it looks about right. You could probably code it in Javascript and call it from within your HTML code if you wished. If you do not know how to code in Javascript, you could probably find a programmer on a website such as FIVERR.COM who could create the code for you without having to spend too much money.

    Enjoy the book!

Hi again,,, the book is ordered :-)
Is it Ok to make a link in my web site to some of yours aticles?
     john frode, December 22, 2014

    Hello John. Yes, of course, you are welcome to put links from your site onto mine. Let me know what your website address is and I'll take a look, as I love to see what other people are doing and your solar cabin in Norway sounds interesting.

My solar system flashes sometimes in afternoon saying the irradance is too low what does this mean as it stops produce power its a new system regards Heath
     Heath, December 23, 2014

    If irradiance is too low it means that your solar array is not generating enough power to run your inverter. This is the darkest time of the year, so the ability to generate electricity from daylight is at the lowest point around now.

    Without knowing more about your setup or where you are based, it is not possible to suggest whether you have a fault with the system or not, but if power generation in general is within your expectations, it is probably simply down to the time of year.

    Now we are past the shortest day of the year, solar production should start picking up quite rapidly again.

I have 4 75 watts solar panels on my boat, right now i have a simple small 30 amp charge control unit. I am looking at getting and installing a Qianqsun® LCD 30A MPPT Solar Panel Regulator Charge Controller 12V/24V 380W/760W With USB. will this help my panels produce more power? is it worth the upgrade? and how many amps are my 300 watts of solar capable of charging at?
i have a 12 volt system consisting of 2 battery banks, each are made up of 2 6v gulf cart batteries linked together to make a 12v system.
thank you for any insite you can provide!

     Jonathan Yoors, December 25, 2014

    If you are running at 12 volts, your solar array is producing peak power of 25 amps. On the face of it, your current charge control unit is sufficient.

    I have not used the Qianqsun® LCD 30A MPPT Solar Panel Regulator Charge Controller, but I have been onto the web and found the instruction manual. It looks like a good unit for the price. Without knowing what your current controller is, I cannot say whether or not the Qiangsun unit is better, but if your existing unit is not MPPT, you are likely to be getting a 20% improvement in energy production with the new controller.

How do we convert kWh/m^2/day to W/m^2. Do we need to multiply 24 hours or the number of hours sun was present? IS there any method?
     Sweta, December 27, 2014

    You are looking for a way of measuring power rather than energy and unfortunately it is not as simple as you suggest. The intensity of sunlight varies throughout the day and because it is also linked with climate, there is no simple conversion calculation.

    If you want an instant measurement of power, you can buy a solar irradiance meter which will give you an instant power reading. If you are looking for a W/m^2 calculator, NASA publish one that gives figures at three hourly intervals throughout the day:

what kind of solar system do I need to keep an 1100 battery pack charged at a 300amp/hour load?
     Terry, December 27, 2014

    Without knowing a lot more about your requirement, it is not possible to give you a proper answer. However, based on what you have told me, you are going to need a much bigger battery pack if your load is a 300 amp continuous output, and you are going to need a very substantial solar array to keep that pack charged.

    Go to the off-grid solar project calculator, fill in the questionnaire and the system will email you a report explaining what you will need.

    Here's the link:

Hello Michael,
I am getting ready to purchase your book, I have an architectural firm in Georgia, I am in the process of getting ready to design and build my personal home for my family of 3 children, 10, 8, and 2. I would like very much to go off the grid completely using solely solar for power. However, I do not want to compromise on my electrical equipment in the home, I want it to be as a normal home with appliances.
Is this possible to do this, or will the battery cost make the system so expensive in upkeep and replacements that it is no longer worth it? (we will use solar in floor hydronic with a propane gas boiler for heating, so no heat pumps etc).
I can control my exposure for maximum gain... Looking forward to hearing from you.
Also, do you design systems, would you be interested in working with me on mine?
What sort of a fee structure do you have?
Thank you for your consideration.
Mark Lytle
President, Unified Design Inc.
     Mark Lytle, December 28, 2014

    Thank you for your message. You have a fun project there! I built my own home about 18 years ago. It is a very rewarding thing to do.

    If you are going entirely off-grid, you need to measure how much energy you are currently using and then identify where you are using it. Get yourself a plug-in watt meter to measure the power requirement of your equipment, then add up how many hours you want to use each item per day. Then go to the Off-Grid Solar Calculator on my web site (, fill in the information and the system will e-mail you a twelve page report that explains the size, configuration and cost of your proposed system.

    I do design systems, but at the moment my time is booked out on working with the White House on renewable energy policy. Lots of fun, but very demanding, and I am not taking on any other work at this time.

    Best of luck with your project.

Dear sir,
how to calculate the solar radiation to irradiance? pls give a brife calculation.
     suresh, December 29, 2014

    Do you mean how to convert solar power to potential energy? If so, there is no brief calculation available as power varies throughout the day based on the intensity of the sunlight.

    If you are interested in some of the calculations that go into solar radiation and irradiance, the Atmospheric Science Data Center at NASA publish their calculations and data, but none of it is brief:

Hi Mr. Boxwell....thank-you for you assistance on my cable splicing question....all I need to get started assembling my system is a pair of batteries and a inverter....first ..Have you ever heard of the VMAXTANK line of deep cycle batteries?/ and what do you think of them ??,,and second ..should I go with a sine-wave inverter ....the Vmaxtank battery is supposedly a military -grade deep cycle battery that can be discharged around 1500 times before replacement....the ones im looking at are 125ah each...I figure with 2 100w panels and a 40amp charge controller I should be good all the 12 volt DC equipment and AC appliances/equipment as well with the inverter.....if any of this is wrong or just plain crazy let me you in advance for any/all assistance
     DAVID HARVEY, December 29, 2014

    I've not used the VMAXTANK batteries, but I am familiar with the technology. The 1500 discharges is based on a 75% minimum state of charge, which means that you should design your system with a four or five day hold-over in order to achieve this in a live situation.

    The cycle life on the VMAXTANK batteries worries me. The 1500 discharges drops to around 400 discharges if you discharge to 25% state of charge. It does depend on the application, but I think that Trojan T105 batteries with a cycle life of around 750 charges to 20% state of charge and almost 2000 on a 75% state of charge would still probably serve you better in the long run, and are cheaper to buy as well.

    Sine wave inverters are definately better than square wave inverters. Some electronic equipment refuses to work with square wave inverters, and the inverters themselves are often not as efficient.

I have a solar panel which puts out 18 volts into a very basic controller. The voltage measured out from the controller is 17.8 volt. I would like to occasionally charge a 12 volt car battery. What is the best and simplest way to drop the voltage out of the controller to about 13 volts? The controller turns on in sunlight and shuts down at night. It was used by drug smugglers to charge their hand held radios and cell phones as they would spend several days on mountain tops reporting on police movements.
Thank you.
     david, December 29, 2014

    The controller will drop the voltage down so that it will charge a 12-volt car battery. Depending on the type of controller will depend on how this does it and whether it impacts on the efficiency of the solar panel.

    If you are buying a new solar controller, buy one that incorporates 'MPPT' as these are the most efficient.

thanks for your book
i live in 35 oroba st, qaluiob 5 minute from shubra el khema in egypt,
i want to know the best direct to panel in this site (in degrees),
i wish to tell me the way u can know the degrees exactly
     ismail gamal, December 29, 2014

    The solar panels should be south facing and mounted at a 60 degree angle from vertical (30 degree angle from horizontal) for best year-round performance.

    To optimise summer performance, the panels could be mounted at an 84 degree angle from vertical (6 degree angle from horizontal). This would maximise power generation during the summer months but would reduce the amount of power that could be produced during the winter.

What is the source for your insolation calculations? NASA? Other?
     Steve Smith, December 30, 2014

    Much of the data source is the Atmospheric Science Data Center at NASA. To this raw data we have added our own calculations to plug the gaps.

hi i am interested in fitting pv solar in my home and was told to fit 2kw system on roofe 35 pitch and the roofe position is facing north west i am a bit surprised about this taught that this position would not be acceptable could you help me out please any other position in house would be problem not enough space for system.i am in ireland.
     alaister draper, January 2, 2015

    As you are in Ireland, you should not fit a solar array to a roof that is facing north-west. You will not generate enough energy to justify the expense.

hey Mike its me again ....I inquired about VMAXTANK batteries and you suggested to me the TROJAN T105 may be better for me ..the battery set-up is going to be inside of my RV and the Trojan T105 is a lead acid where the VMAX is a sealed batt...and there are shipping restrictions on lead acid batteries that have to have their hydro replenished....meaning I would get the trojans from FED-EX but they would be dry and need filling with distilled water and charged...not to mention the batts are going to be charging inside the RV giving of gases(which im trying to eliminate...and the Trojans are 6 ok with trying 6 volt batts but I do not know how to wire them up (series or parallel) original plan was 2 deep cycle batts being supplied by 2 solar panels(100w) then going to an inverter(2000w)for running a energy saver flat screen and a lap-top..I have a basic layout of what cable/wiring/fuses to use...but now I hit a wall.....any suggestions my friend ??

     DAVID HARVEY, January 3, 2015

    Fair point, if you are installing the batteries in an RV, you'd need to have a ventilation system for releasing the gases from the batteries. I don't have experience of the VMAXTANK batteries, but looking at the specification they should be pretty good. The other alternative, which is more expensive but are the 'Rolls Royce' of sealed batteries are the Optima Yellow Tops.

    As you are running a 2kW inverter, I would suggest that you run at 24 volts. Running at the higher voltage will keep your system running better with the drain that you are putting on your system.

    That said, if you are only running an energy saver flat screen and a laptop, you're probably going overkill with a 2kW inverter. Consider choosing a flat screen that can run at a lower voltage (12 volts) and using a voltage converter for the laptop and eliminating the power supply. Not only will that be cheaper than fitting an inverter and will be far more energy efficient.