Electronic – Sizing a battery according to an existing solar panel (and not the opposite!)

battery-operatedcapacitysolar energy

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?

Best Answer

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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