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I have one 60-cell 260Wc ~30V ~8.5A panel left over after a solar installation.
I did record consistent and factual 1400Wh per day and per panel in winter.

I want to use the energy of this extra panel for a home-made low-voltage ventilation in our house (12V ideally, or 24V more realistically(?)).

  • In winter I can use the "real time" production and use any surplus to heat the vented air itself.
  • But in summer, I would like to draw the maximum of daily-stored energy to vent the house with cool air during the night.

I have no need for cycles larger than 24h... but I still need a battery.

So here is the question: I want to store the best of a 1400Wh daily solar production into a battery. Which capacity should I get?

As you see I am not driven by my power requirement, but by what I can already produce with this panel. It stands out from what is usually talked about and what solar calculators do (namely: assessing needs to know the hardware to buy).

Given "rules of thumbs" like "15 watts delivers about 3,600 coulombs per hour of direct sunlight (i.e. 1 Ah)":

1400Wh in a 12V battery = 116Ah
1400Wh in a 24V battery = 60Ah

I was to get a 60Ah, 24V battery setup (e.g. 8 LiFePo cells) but I realized I am probably oversizing it given losses in the changer, BMS (battery management system) and battery charging chemistry itself?

I am unsure that the above "rule of thumb" takes these issues into account. Can someone help me?

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  • \$\begingroup\$ You also might want to take into account that some batteries like to be not deeply discharged, or charged, so you might want to keep them betwenn 40%-80% or whatever is best for them. \$\endgroup\$ – PlasmaHH Feb 1 '17 at 11:18
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You actually want to oversize even more than that. Here are some points to consider:

  • Lithium-based batteries have shorter lifespan with 100% depth of discharge. If you want your installation to work for >10 years, it's recommended to keep cell in their comfortable voltage range (3.0 ~ 3.3 V), without ever going as low as 2.5 or as high as 3.65V. This means you will be using about 80% of their capacity.
  • Even in the best conditions, you should expect available capacity to decline. Depending on the quality of cells, you'll end up with 80% of original capacity in 10-20 years, assuming 1 charge cycle per day.

So, if you need 60Ah of useful capacity in your system 10-20 years from now, you should aim at 60/(80% * 80%), or at least 93Ah of raw capacity. That's how I would size it.

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  • \$\begingroup\$ Thanks! I tend to over-engineer everything, but I do understand the point. Not being able to "fully" charge the battery during the day is in fact a safety, instead of looking as "not optimal". Esp. I read that LiFePo are OK/better with partial charge, so there would be no loss by being too big. But isn't the "rule of thumb" well too optimistic to compute the "unsafe" battery capacity? I doubt that 1Wh of solar production makes 1Wh in the battery! \$\endgroup\$ – MoonCactus Feb 1 '17 at 13:21

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