Panels needed to charge a 24V 300Ah battery bank

[rmaddy]

I'm getting confused as a solar newbie trying to work this out.

Let's say I plan on having six 12V 100Ah LiFePO4 batteries wired as 2S3P (I think) resulting in a 24V 300Ah bank. Since the batteries are really 25.6V that's a total of 7,680Wh. Assuming 6 hours per day to charge, I need 1280W of solar panels. Let's call it 1200W. Correct so far?

In my research I have seen a rule of thumb stating you want 2W solar for each Ah of battery. That seems right with all of the 12V system examples I've seen. For example, charge a 12V 100Ah battery with two 100W 12V panels.

But I'm planning a 24V system. Since I will have 300Ah (at 24V), do I need 600W of 24V panels or 1200W of 12V panels? But watts are watts, regardless of volts, right? My original math states 1200W. Where I'm really confused is 12V versus 24V solar panels. Could I use twelve 100W 12V panels? What about four 300W 24V panels? Or would I just need two 300W 24V panels since that meets the 2x rule-of-thumb I've seen. Ouch, my head hurts now.

I guess the more direct question boils down to this:

Do I need two or four 300W 24V panels to charge a 24V 300Ah battery bank given the typical average of 6 usable hours of sun per day?

 

[Ped]

2w per amp hour is for lead acid. (And its actually more like 4-6:1 depending on load and type). With lifepo4 you more or less only need to replace what you take out because they dont need to be perfectly 100% recharged and have much higher charge efficiency.

 

[rmaddy]

The 2:1 I've seen was for LiFePO4 based on 6 hours of sun and a 12V system. But let's ignore that rule-of-thumb and do the proper calculations. And that is where I need help. Thanks.

 

[Ped]

Its all dependant on your load. Figure highest draw at lowest sun hour time of year and give her a little extra for good measure.