How it works
A battery stores energy equal to its voltage times its amp-hour rating, in watt-hours. Only part of that is usable: most batteries should not be fully discharged, and an inverter loses some energy as heat. Multiply by those fractions, then divide by the load in watts to get hours.
Real capacity falls at high discharge rates and low temperatures, and the nameplate Ah is usually quoted at a slow rate. Treat the result as an optimistic estimate and test your own setup.
Worked example
A 12 V, 100 Ah battery, 50% usable, 90% inverter efficiency, running a 100 W load:
- Stored energy = 12 V × 100 Ah = 1,200 Wh
- Usable energy = 1,200 × 50% × 90% = 540 Wh
- Runtime = 540 Wh ÷ 100 W = 5.4 hours
| Input | Value |
|---|---|
| Battery voltage | 12 V |
| Capacity | 100 Ah |
| Usable share of the capacity | 50 % |
| Inverter and wiring efficiency | 90 % |
| Load | 100 W |
| Result | Value |
|---|---|
| Runtime | 5.4 hours |
| Usable energy | 540 Wh |
| Total stored energy | 1,200 Wh |
| Battery current at this load | 9.3 A |
Assumptions and limits
- Constant load and constant voltage; real batteries sag as they discharge.
- The usable share is your input from the battery maker. This tool does not know your chemistry.
- Capacity falls at high discharge rates (Peukert effect) and in the cold, which the tool does not model beyond a warning.
Common questions
How long will a 100Ah battery last?
At 12 V, 100 Ah is 1,200 Wh; at 50% usable and 90% efficiency, about 540 Wh, or 5.4 hours at 100 W.
How do I convert amp-hours to watt-hours?
Multiply by the battery voltage.
How much of a battery can I use?
It depends on the chemistry and the maker’s recommendation, so this is an input.
How do I size a whole system?
Use the solar and battery sizing calculator to size the bank and the array.
Sources
- Battery energy E = V × Ah; runtime = usable energy ÷ power.
Updated 2026-09-30