Solar Battery Bank Calculator
Size a battery bank for a solar system: amp-hours, and how many batteries in series and parallel.
Bank size
370 amp-hours
- Batteries in total
- 8
- In series per string
- 2
- Strings in parallel
- 4
- Installed energy
- 9,600 Wh
You need 8 batteries: 2 in series and 4 strings in parallel.
Calculation breakdown
| Step | Formula | Result |
|---|---|---|
| Bank amp-hours | 2,000 Wh × 2 days ÷ (24 V × 50% × 90%) | 370.4 Ah |
| Batteries needed | 24 V ÷ 12 V in series, 370.4 Ah ÷ 100 Ah in parallel (rounded up) | 8 |
How it works
Enter your daily energy use, the system and battery voltages, and the battery size.
Worked example
2,000 Wh a day, 2 days of backup, a 24 V system, 12 V 100 Ah batteries, 50% depth of discharge and 90% efficiency:
- Bank size: 2,000 × 2 ÷ (24 × 0.5 × 0.9) = 370.4 Ah
- In series: 24 ÷ 12 = 2
- In parallel: 370.4 ÷ 100 = 3.7, rounded up to 4, so 8 batteries in total
Sources
Frequently asked questions
How do you size a solar battery bank?
Multiply your daily watt-hours by the days of backup, then divide by the system voltage, the depth of discharge and the efficiency. That gives the bank size in amp-hours.
How many batteries do I need in series and parallel?
Batteries in series add voltage, so divide the system voltage by one battery's voltage. Batteries in parallel add amp-hours, so divide the bank amp-hours by one battery's amp-hours and round up.
What depth of discharge should I use?
It depends on the battery type and the maker's advice. The 50% default is an example, since Battery University gives cycle life against depth of discharge but no single default.
How many days of backup do I need?
That is your choice, based on how many cloudy days you want to ride through. The calculator uses 2 as an example.
Why can't I use any battery voltage?
The system voltage must be a whole multiple of one battery's voltage so the batteries can be wired in series. A 24 V system with 12 V batteries needs 2 in series.
An estimate for information only. Days of backup, depth of discharge and efficiency are your own inputs; check your battery maker's limits.