appropriate generators for off-grid photovoltaic system

2 Posts
Dec 13, 2010 02:11 pm
appropriate generators for off-grid photovoltaic system

I would like to add a small back-up generator to my 1500 watt,220 AH(AGM)system, for getting through the rainy season with some dignity.I'm using a Xantrex TR2424-120-60 inverter/charger. What is my best/safest option in a generator? Thanks, Ed Spencer
« Last Edit: Dec 13, 2010 02:18 pm by Edward Spencer »
39 Posts
Jan 23, 2011 03:03 am
Re: appropriate generators for off-grid photovoltaic system

Hey there, that's my situation too!  I've got a 640W 600Ah off-grid PV system, wanting to hook up a generator without frying my Xantrex XW60 MPPT solar charge controller.  Getting nowhere on Xantrex's website, for it all seems to be geared at grid-tied systems, with NO MENTION of us off-grid PV system users. I don't get it.
351 Posts
Jan 24, 2011 04:01 pm
Re: appropriate generators for off-grid photovoltaic system

You seem to be asking an entirely different question from Ed's. He apparently read the installation manual for his inverter and was asking what the smallest generator is, that he can use.  That recommendation should come from Xantrex.

You did not mention what your inverter is. The answer of how to hook up the generator should be in the installation manual for it.
49 Posts
Jan 25, 2011 05:10 pm
Re: appropriate generators for off-grid photovoltaic system

Im using a home Kohler 12kw that is propane and the xantrex inverter turns it on when needed to charge the battery bank ,a 48v system.  The kohler warranties it on solar system and Generac does not, that was my first choice but I am happy with the Kohler.
99 Posts
Jan 26, 2011 12:33 pm
Re: appropriate generators for off-grid photovoltaic system

I just installed a backup generator on my system.  I decided off the bat that I would not risk my solar or wind chargers or try to use my inverter as a charger.  The generator should have its own independent charger.  I want to avoid single points of failure like the plague.  What if your solar charge controller broke and that's why you need the backup power source?  Too bad, now they're both off line. 

Also, my Outback inverter treats a backup generator as a whole-house grid-replacement solution, disconnecting the batteries from the process.  This is not what I want at all.  If I don't get enough wind or sun and my batteries start getting low, say 22V, I don't want to disconnect my system and start running a generator for everything.  All I need is a low-power source to start charging my batteries.  If my well pump comes on, calling for 1500W, I don't need my generator to cover that full amount since the batteries still have juice in them.  The batteries can cover that load for the 3 minutes it's on, and then my generator can recharge them for the next 5-6 minutes to recover that amount of power, and then continue charging back up to float voltage.

My house is budgeted to between 6 and 10kW per DAY.  A 12kW generator would be well beyond overkill.  Oversizing a generator is not a good idea because you'll waste fuel and probably have more maintenance and a shorter generator lifespan.  Generators operate most efficiently when loaded at 50-100% of their rated output power.  You also have to ensure that your generator and charge controller are big enough to efficiently charge your batteries.  Somewhere between 5% and 20% of your batteries' rated amp-hours will both charge in a reasonable amount of time and also prevent overheating.  At only 220Ah, you could get away with a 15-40A charger and generator.  One thing to keep in mind is that chargers and generators are sometimes rated by their 120VAC input/output, so you'll have to convert to your DC voltage to figure out actual amps to your batteries.  E.g. a 20A/2400W generator will put out 200A (minus losses) at 12V or 100A at 24V.

Another thing to consider is your fuel type.  Diesel is often used because the engines tend to be simpler and more reliable.  However, you will most likely need your backup generator in the depths of winter when diesel engines are difficult to start.  Gasoline generators are often cheap up-front, but reliability and maintenance can be significant.  If you go months between uses, the gas can gum up and ruin your generator.  Also, liquid fuels are prone to spills and require that you frequenly refill the generator's tank.  For these reasons, I went with propane.  Since I already use propane for cooking and back-up heat, I already have a 100 gallon tank.  Propane will not spill ever (even if you could pour it on the ground, it would evaporate before it got there).  It starts easily in cold weather.  And it's very clean-burning and will not gum up, ensuring a long generator life.

Something else to keep in mind is if you want your generator to auto-start given a signal by your inverter, charge controller, or other sensor.  It sounds nice, but do you want to pay the hefty premium for such a generator?  I decided no.  If I'm only going to need to run it a few times a year, I can keep an eye on the system voltage and go out and manually start it when needed.  If you were going to need to start it daily, then that would be another issue.  I also have a voltage meter which will put out an audible alarm (like a low voltage chirp on a smoke detector) if it gets too low, so I won't be surprised with the inverter cutting out due to low voltage.

So in the end, I went with a 2000 Watt Sportsman Propane Generator (rated at 1300W continuous) from for $299.00 and an Iota 24V 40A Smart Battery Charger from for $365.00.  The total cost after shipping, wires, fuse, and propane hookup was under $750.  The combo puts out nearly 1kW, which is the rated output of my wind turbine at full speed or the average output of my PV array on a partly-sunny day.  In other words, plenty to charge my batteries -- 7.2% of my battery bank's rated Ah.  The charger puts a roughly 75% load on the generator, making it an efficient use of fuel.  It consumes about a third of a gallon of propane per hour.  That means that you could run the charger for 15 hours on a 20-lb tank or 300 hours on a 100-gallon tank.

I've only used it once so far (and didn't really need to) -- it brought my batteries from 23V to 26V in a matter of minutes and went to float mode within a half hour.  I have it in a small well-ventilated shed along with my propane tank, which backs up to my power shed containing my batteries and stuff, and you can barely hear the generator from a few dozen feet away. 

This generator is actually meant for taking camping and comes with a regulator for a 20lb (5-gallon) tank like used on BBQ grills.  So even if you don't have a 100-gallon tank like I do, you can still use it and buy the fuel at almost any gas station or home improvement store (though at very low temperatures, the smaller tank may have trouble vaporizing enough to keep the pressure up).  The generator is quite small and portable.  I used copper pipe clamps to secure it to a table at a comfortable height to start it.  It easily started below freezing. 
99 Posts
Jan 26, 2011 01:24 pm
Re: appropriate generators for off-grid photovoltaic system

These tri-fuel hybrid generators are interesting:

This is a good discussion of generators in PV systems from Sandia National Labs:
220 Posts
Jan 30, 2011 09:29 pm
Re: appropriate generators for off-grid photovoltaic system


 thank you for sharing your thoughts on your choice for a generator for your system and the reasons behind those choices.

excellent research and logical reasoning!

cheers, dave


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