Battery Runtime Calculator
Enter battery Ah and voltage, your load in watts, plus DoD and inverter efficiency, to get realistic runtime — not the naive Ah ÷ A figure.
Last updated: 2026-09-15
How the calculation works
- Nominal energy is Ah × V. DoD limits how much you can draw before recharging; inverter efficiency subtracts conversion losses.
- Runtime is usable energy divided by the load — and the tool also shows what the load consumes per day for solar sizing.
Formula
Usable Wh = Ah × V × DoD × η_inv Runtime (h) = Usable Wh / Load W
| Symbol | Meaning | Unit |
|---|---|---|
Ah | Rated capacity | Ah |
DoD | Depth of discharge | — |
η | Inverter efficiency | — |
Worked example
Interpreting the result
The difference between naive and realistic runtime is entirely DoD and efficiency. Draining lead-acid below 50% repeatedly shortens its life dramatically, which is why the DoD cap matters more than the capacity figure. Lithium (LiFePO4) tolerates 80–90% DoD, effectively doubling usable capacity per rated Ah.
Assumptions
- Constant load power, battery at room temperature.
- Peukert effect ignored — accurate for lithium, optimistic for lead-acid at high discharge rates (above ~C/2).
Limitations
- Lead-acid capacity drops sharply at high discharge rates and low temperatures.
- Inverter no-load consumption (10–30 W) is not subtracted — it matters for small loads over long periods.
- Battery capacity degrades with age; a 5-year-old battery may hold 70–80% of rated Ah.
Frequently asked questions
How long will a 100Ah battery run a fridge?
A typical 12 V fridge averages 40–60 W (cycling). At 50 W with a 100 Ah AGM battery at 50% DoD and 90% inverter efficiency: about 10 hours. With LiFePO4 at 90% DoD: ~19 hours.
Why not use the full battery capacity?
Lead-acid chemistry degrades when deeply discharged — staying above 50% DoD multiplies cycle life several times. Lithium tolerates deep discharge much better.