For a fridge, enter the compressor watts and the hours it actually runs (often a third of the day), or use the kWh per year on its energy label ÷ 365.
How the battery bank calculator works
BatteryBankCalc adds watts × hours × quantity for every load to get watt-hours per day. Battery watt-hours = daily Wh × days of autonomy ÷ (usable depth of discharge × inverter efficiency), and amp-hours = that ÷ system voltage. Batteries in series = system voltage ÷ battery voltage; strings in parallel = amp-hours needed ÷ battery amp-hours, rounded up. Inverter = sum of all load watts × (1 + headroom), rounded up to a standard size. Solar array = daily Wh ÷ inverter efficiency ÷ (peak sun hours × system efficiency); controller amps = installed panel watts ÷ system voltage × 1.25.
Usable capacity by battery type
| Battery | Typical usable depth of discharge | Usable Wh from 12 V 100 Ah |
|---|---|---|
| Lithium iron phosphate (LiFePO4) | 80% | 960 Wh |
| AGM / gel lead-acid | 50% | 600 Wh |
| Flooded lead-acid | 50% | 600 Wh |
Batteries in series for each system voltage
| System | 12 V batteries in series | Current for a 1,000 W load |
|---|---|---|
| 12 V | 1 | 83.3 A |
| 24 V | 2 | 41.7 A |
| 48 V | 4 | 20.8 A |
Depth of discharge, inverter efficiency (90%), solar system efficiency (80%) and the 1.25 controller factor are common planning figures, not product specifications; follow your battery, inverter and controller manuals, and size cables and fuses for the current. A licensed electrician should connect any system to house wiring. Results are estimates.
Frequently asked questions
How many batteries do I need for solar?
Daily watt-hours × days of autonomy ÷ (depth of discharge × inverter efficiency) ÷ system voltage gives the amp-hours. For 2,150 Wh/day at 24 V with LiFePO4 that is about 124 Ah, or four 12 V 100 Ah batteries (2 in series × 2 in parallel).
What size battery bank for 2 kWh per day?
About 2.8 kWh of LiFePO4 at 80% depth of discharge and 90% inverter efficiency for one day, or about 4.4 kWh of lead-acid at 50%.
Should I use 12, 24 or 48 volts?
Higher voltage means lower current, thinner cables and smaller charge controllers. 12 V suits small RV and van systems, 24 V mid-size cabins and 48 V home systems.
Is BatteryBankCalc free?
Yes, with no sign-up or ads.
Are my loads saved?
Yes, in this browser only.