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.
Open calculatorLast 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.
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.
Runtime in hours of a 12 V battery at 50 % usable capacity
Inverter efficiency is assumed to be 90 %.
| Load | 50 Ah | 100 Ah | 200 Ah |
|---|---|---|---|
| 50 W | 5.4 h | 10.8 h | 21.6 h |
| 100 W | 2.7 h | 5.4 h | 10.8 h |
| 200 W | 1.4 h | 2.7 h | 5.4 h |
| 500 W | 0.5 h | 1.1 h | 2.2 h |
| 1000 W | 0.3 h | 0.5 h | 1.1 h |
Runtime in hours of a 12 V battery at 80 % usable capacity
Inverter efficiency is assumed to be 90 %.
| Load | 50 Ah | 100 Ah | 200 Ah |
|---|---|---|---|
| 50 W | 8.6 h | 17.3 h | 34.6 h |
| 100 W | 4.3 h | 8.6 h | 17.3 h |
| 200 W | 2.2 h | 4.3 h | 8.6 h |
| 500 W | 0.9 h | 1.7 h | 3.5 h |
| 1000 W | 0.4 h | 0.9 h | 1.7 h |
Why runtime is shorter than amp-hours divided by amps
A 100 Ah, 12 V battery stores 1.2 kWh on paper. Only part of that can be used: at 50 % depth of discharge the usable energy is 0.54 kWh, and an inverter at 90 % efficiency wastes some of it. A 100 W load therefore runs for 5.4 hours, not the 12 hours the nameplate suggests.
Choosing the usable fraction
Lead-acid batteries are commonly limited to about 50 % discharge to protect their life, while lithium iron phosphate batteries usually allow 80 % or more. The right figure is the one in your battery's datasheet, so check it before you rely on the tables above.
What the calculator does not model
- Heavy loads shorten the effective capacity of lead-acid batteries (the Peukert effect).
- Cold temperatures reduce available capacity.
- Capacity falls as a battery ages, so add margin for the end of its life.
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.