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Get Powered Up! Certified Energy Manager Jeff Yago answers your alternative energy questions

Wondering about a great new energy-saving device
you found on the Internet? Then CLICK HERE!

Sorry. Jeff no longer answers questions online.
This will remain as a searchable
resource for all BHM website visitors.


Designing an off-grid PV system

Saturday, May 9th, 2009

Hi Jeff

I am an electrical engineer and received this project to design an off grid PV system ( I think also known as a stand alone system since the power grid is not involved). I have never done anything similar in my classes that had to do with solar panels and PV systems in general.

I would appreciate any guidance that you can give me regarding the basic steps or important calculations to calculate in order to get started with this designing. I have attached the project question to this email.

Thanks for your help, I just need guidance to get started since I have never ever heard of PV systems before in my life.

I appreciate your help



As noted on this web site, we cannot provide specific design information on any solar project due to limited time available to answer each question on a free site, and with any design there could be critical information we do not have that could affect the advice given.  However, I always try to provide a direction for our readers when possible.

First, any off-grid system starts with identifying what electric loads are to be powered, and for how many hours per day.  Your list of this equipment has some serious problems which need to be addressed before even starting to complete a solar design.  For example, you list an electric stove which is almost never powered from a solar system due to high load, but it is OK to power a microwave oven since they only operate a few minutes.

You list running power tools each day, but make no mention of a well pump which we would expect to have on an off-grid rural installation.

You list a refrigerator as a constant 24 hour per day load, but refrigerators and freezers cycle on and off all day, so you need to find the daily kW-hour usage for your average room temperature to use and not an instantaneous  watt value.

You list a solar array output of almost 5 kWhs per day per kW installed which is highly doubtful.  This may be possible for peak summer sunny days, but this is not realistic for an average output per day.  We typically see solar arrays producing only 50 to 70% of their “nameplate” ratings except for very brief periods of extreme cold sunny days, and blue sky noon summers days with mild temperatures.  For off-grid designs, this is a critical factor since you have no grid to fall back on.

Your battery bank requirements are not realistic and must have come from somebody reading a book.  For example, your daily energy loads total 8.9 kWh and you want 3 days of no-solar battery time with 50% discharge.  This means the battery bank would need have a capacity of 53.4 kWh, which would require four (4) deep cycle 1200 amp-hr batteries @ 12 volts each in series, which would have a combined weight of 4,640 pounds and would cost over $10,000 just for the batteries, plus a pot load of shipping and un-loading costs.

Finally, there are serious electrical design considerations when designing any solar power system, and this is even more serious for an off-grid system.   The National Electric Code has an entire Section 690 that addresses wire and fuse sizing for solar arrays and I strongly suggest that you either obtain the services of a solar designer that has experience with these systems or plan on taking several courses on the subject first.

Good luck, and be sure to include a good fire extinguisher with your installation if you do not follow my advice.

Jeff Yago


Double duty for a Honda/Yamaha inverter generator

Wednesday, July 23rd, 2008


I have a remote cabin that I am currently running off of a Honda EU3000is generator. I am planning on adding a battery bank, inverter and solar panels at a later date as funds allow. In order to save money now, I was thinking that I could buy the battery bank and hook up the battery bank to the inverter on the EU3000. Why not use the EU3000 inverter for double duty? I could get 110V pure sine wave power off of the bank or the generator.

There would still be the issue on charging the battery bank (you would need to purchase an appropriate charger for the bank) – could you charge the batteries from the EU3000 and run the EU3000 to provide 110V power simultaneously. I would then add the proper pure sine wave inverter to handle the solar panels, charging the bank from the EU3000 and to provide 110V power.

Too complicated? Not any monetary savings? Crazy idea? How would you go about engineering the system so you didn’t hurt anyone or damage/destroy any electrical equipment?


Paul Chan


This is a standard layout for a generator based off-grid homes without solar. I would match this with a Xantrex 4024 or an Outback 3624 inverter if you plan to expand later, but these are a little large for your 3 kW generator, and you will need to adjust the battery charger setpoints or they will overload the generator. In addition, these inverters can be programmed to “assist” a generator which means during battery charging and also supplying other 120 volt loads from the generator, if the generator demand goes above setpoint, the inverter will first back off the charging to reduce the load.

If the load still is increasing (like starting a well pump for a few minutes) the inverter will take some battery power back and “assist” the generator to avoid over-loading. You can even program either of these inverters to start the generator when the battery charge level drops below a minimum setpoint.

For this type system, its best to operate your LARGEST loads while the generator is running and also charging the batteries, instead of first charging the batteries then stopping the generator and powering the same loads from the inverter. Since there is a loss of efficiency during the charging process and again during the inverter process, large loads are better operating only when the generator is running and supplying the power directly.

Since the solar array will have its own solar charge controller, this will be totally separate charging system, although it will be charging the same batteries so you can add the solar later. You can also add solar modules a few at a time since they are expensive, but get the charge controller that will be large enough from the start so you don’t have to keep upsizing.

Good Luck,

Jeff Yago



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