Solar Expert Posts: 630 ✭✭
I am upgrading from my 'toy' power pack. I wanted to do it big, but since my g/f lost her job in time for the holiday my budget shrank.

I bought an 80 watt, laminate panel (cheap) and I want to match it with an appropriate deep-cycle battery. It's specs are,
80W
18.0 Voc
4.40 Isc
16.20 Vmp
3.96 Imp

On average throughout the year I have 4 good sun hours a day. Using the fast formula I've read elsewhere in these forums, I cut my wattage in half, then multiply it by 4, for 160 watts.

What is the next Math I need to match the 5-13% charging range for a proper battery?

Thank you.

Roughly for a 12 volt battery bank using the 5-13% rule of thumb:
• 80 watts * 1/14.5 volts * 0.77 derating * 1/0.13 rate of charge = 33 AH
• 80 watts * 1/14.5 volts * 0.77 derating * 1/0.05 rate of charge = 85 AH
The Vmp is a bit low (and Voc is surprisingly low)--So in very hot weather, the panel may not produce enough voltage to quickly recharge the battery (by the time controller and wiring voltage drop is taken into account).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 630 ✭✭

Thank you BB

I got what I paid for. It'll only get used a few hours a month, so there are long stretches for recharging. I was figuring a 65 AH rating based on the 85% efficiency of my controller(all my wire runs are less than 3 feet). I'd rather get an 85 AH battery for future upgrade.

Thanks for writing out the math. I was searching for this through several threads last night, and didn't hit any upon any of them