# Proper amount of solar panels needed to charge batteries

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.

• 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. Nov 20, 2019 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? Nov 20, 2019 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... Nov 20, 2019 at 20:18
• You seem to know how series and parallel work with batteries, do you doubt solar panels work the same? Nov 20, 2019 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. Nov 20, 2019 at 20:35