Real runtime is shorter in the cold and at high discharge rates, especially with lead-acid batteries.
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 long will a 100Ah battery last?
A 12.8 V 100 Ah LiFePO4 battery stores 1,280 Wh; at 80% usable and 90% inverter efficiency that is about 920 Wh, enough to run a 100 W load for about 9.2 hours.
How long will a 12V battery run a 1000W inverter?
At a full 1,000 W load a 100 Ah LiFePO4 battery lasts under an hour (about 0.9 h at 80% usable and 90% efficiency).
Why use only 50% of a lead-acid battery?
Deep discharges shorten lead-acid life sharply; stopping at about 50% is the usual compromise.
Is BatteryBankCalc free?
Yes, with no sign-up or ads.
Are my loads saved?
Yes, in this browser only.