# A simple question, I think...

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Registered Users Posts: 11
However, I am not having a lot of success finding the answer. Ok, I am off grid in a great house and thriving.

We started this experience in a trailer with an Outback inverter driven by a windmill, then grew to a few solar panels with outback controllers working well. All of this was supported by the typical golf cart battery (eight batteries) 24 volt system.

We then built the house and it works well on a separate system (previously described somewhere around here).

The original trailer system is now supporting my wife's studio/greenhouse. We added to the solar panels (we now have 10 180 Watt Panasonic panels) over the years. Of course, now, the batteries are in float probably 2 hours into the solar day. So...by next summer it is gonna be time to update the batteries, anyway. I'm trying to figure backwards...I have the solar panels in and running...we live in Rowe, NM, with probably as good of sun as you can get in the continental US...how much of a battery bank can I support? Price is not terribly important, but I do want to stay with the same CC's, inverter and panels...so 24 volts. Really, this isn't a really heavy use (potter's wheel, a few lights, a propane heater) set-up. We are gonna be running a kiln...but we have a separate dedicated generator for that (she doesn't like gas kilns).

Suggestions?

Jim

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Re: A simple question, I think...

So the question is: how large of a 24 Volt battery bank can 1800 Watts of array on an MPPT controller support?

Well ...

1800 * 0.77 = 1386 / 24 = 57 Amps peak output current. That would be about 570 Amp hours @ 24 Volts, or a lot of battery.

That's based on 77% typical efficiency, a 10% peak charge rate, and 25% DOD.
• Solar Expert Posts: 119 ✭✭
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Re: A simple question, I think...

Well you have 1800w x .77 at 24v is about 58amp of charge current. This would support up to an 1100ah battery at 5% charge current which is the minimum recommended around here. There are many other variables that need to be looked at. You already know that you are doing this backwards so I wont repeat it, but if you draw these batteries down to 50% dod you will not be able to charge them in one day. On a good day these panels could generate around 5.5kwh of power and your batteries could hold 26kwh of power. At 50% dod that is over 13 kwh that needs to be replaced.
Ned
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: A simple question, I think...

generally, we recommend that a battery receive between a 5% and 13% charge rate. now pvs are rated at stc so this is not typical of what one may see which is roughly 77% of that stc rating. 1800w stc x .77 = 1386w. 1386w divided by 24v (battery voltage nominal) = 57.75a. you have roughly 440ah of battery capacity now for a tad over 13% rate of charge which is fine to go with. if loads are on during the charging process this will take away from the charge to the batteries so be aware of that and the net effect to the batteries. i would advise to try and keep the rate of charge at least at a 10% rate. if you get the agm style batteries you may opt for a bit more capacity overall as the agm batteries are more efficient than fla types by roughly 10%, but at a higher overall cost.

what i would do if replacing the battery bank might be to get l16 style batteries with about the same ah rating as the combined strings you already have are so that you won't need more than one string of batteries. many choices and things for you to weigh out in your considerations. if you have found that you do need more battery capacity than you have then you may need to upgrade that and the pvs as well. a higher rating may keep you locked into a 2 string battery bank depending on the current ratings of the batteries available. it's a juggling act sometimes.