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

Question

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?

Answer 1

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.