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I am working on a plan for a 24V electrical system. Based on my needs I plan to have six 12V 100Ah LiFePO4 batteries wired up (2S3P I believe) to give me 24V 300Ah.

Assuming I need to fully recharge this battery bank each day and going on a rough average of 6 hours of solar charging per day, the math seems to indicate I need roughly 1200W of solar panels:

25.6V * 300Ah = 7680Wh
7680Wh / 6h = 1280W (let's call it 1200W)

I see that there are 12V solar panels and 24V solar panels. But am I correct that the panel voltage is somewhat irrelevant. Do I need 1200W regardless of the panel voltage?

Would using twelve 100W 12V panels work? Then what about using 300W 24V panels? Would I need 2 or 4 of those panels? I'm hoping the answer is 2 panels but I suspect it's 4 since it is the wattage, not the voltage that matters, correct?

To summarize, would I need two or four 300W 24V solar panels to recharge a 24V 300Ah battery bank on a daily basis assuming 6 hours average of useful solar energy?

If more specifics are needed about the batteries or solar panels, I can update with details. Just let me know what is needed.

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    if you use an mppt controller, only watts matter. if you want year-round power, you should double your estimates, you can run AC in the summer to utilize the un-keepable surplus. You'll need that extra capacity to produce your desired amount in december on cloudy weeks. – dandavis Nov 20 '19 at 19:54
  • @dandavis Assuming I do use an MPPT controller, what is the answer to my question? Would I need two or four 300W panels and would that hold whether the panels were 12V or 24V? – rmaddy Nov 20 '19 at 20:10
  • You can ignore volts on the panels if they give watts. If you're not tracking the sun, even 4 300w panels will leave you without power some of the time. A well-placed 300w panel only delivers a true 300w for a small fraction of the year; cloudless noontime around summer solstice. You might get only 700wh/day from a 300w panel in December. You can choose between a generator and panel over-kill (10+ panels) if you need reliable power. I know it's a bummer, but physics doesn't care... – dandavis Nov 20 '19 at 20:18
  • You seem to know how series and parallel work with batteries, do you doubt solar panels work the same? – Harper - Reinstate Monica Nov 20 '19 at 20:25
  • @Harper-ReinstateMonica I guess I'm just thrown a bit by 12V vs 24 solar panels. II get that 1200W is 1200W regardless of the voltage. I guess I'm fighting reality with desire. I want the math to work out that I can recharge my 24V 300Ah battery bank with two 300W 24V panels. But It seems to work out to four, not two. I'd be OK with needing four 300W 12V panels because the answer here affects what MPPT charge controller I would need. Four 300W 24V panels (in series) require a very expensive charge controller. – rmaddy Nov 20 '19 at 20:35
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I'm a little concerned with sizing here, but your math seems pretty solid. 1200W is 1200W, and the voltage of the battery doesn't change that.

So yeah, if you want to charge a 7200WH battery pack in 6 hours, that is indeed 1200W. You're not going to get that out of 600W of panel, unless you wait twice as long.

Of course, you won't get 1200W out of a 1200W panel set except when it's square-on with the sun.

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  • It's not the voltage of the battery that had me second guessing the math, it's the voltage of the panels. I just had this hope that the panels being higher voltage would somehow change the answer even though I know it's the watts that matter. Thanks. Can you give me a hint that will fit in a comment regarding your sizing concern? – rmaddy Nov 24 '19 at 15:35
  • 6 hours of sun a day in the winter is highly optimistic. Running a battery from 100% to 0% every day is not workable. These align pretty closely with mistakes of assumption I often see made by people new to solar, so I wonder if other mistakes may be present. – Harper - Reinstate Monica Nov 24 '19 at 15:46
  • I understand that the 6 hour number is an average over a year and it will be higher in the summer and shorter in the winter. For me that is fine. My needs will be met in the summer and my usage will be much less in the winter so that should work out for my needs. – rmaddy Nov 24 '19 at 16:44
  • You weren't specific, so I gathered it was something like that... Just y'know, wouldn't be doing my job if I didn't mention it :) – Harper - Reinstate Monica Nov 24 '19 at 17:07
  • Glad you brought it up. – rmaddy Nov 24 '19 at 17:18

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