How to use the battery runtime calculator
- Enter the capacity from the battery label, in Ah, mAh, Wh or kWh. For a bank of identical batteries, enter one battery and set the number of batteries and how they are wired.
- Set the voltage and battery type. The type fills in a sensible usable capacity and Peukert exponent, which you can change under Advanced settings.
- Enter the load in watts: the total of everything running at once. Tap a preset for a typical appliance, or switch to amps for a DC load.
- Choose AC or DC. Appliances plugged into an inverter or power station lose about 10% to conversion; 12 V devices wired straight to the battery do not.
The result updates as you type. Use Copy link to save or share the exact setup.
The battery runtime formula
For most batteries and everyday loads, this simple version is accurate to within a few percent:
Runtime (h) = Ah × V × usable % × efficiency ÷ load (W)
Worked through for the default example, a 12 V 100 Ah LiFePO4 battery running a 100 W load through a 90% efficient inverter:
- Energy: 100 Ah × 12 V = 1,200 Wh
- Usable energy at 90%: 1,080 Wh
- Power taken from the battery: 100 W ÷ 0.90 = 111 W
- Runtime: 1,080 Wh ÷ 111 W ≈ 9.7 hours, trimmed to 9.4 hours once Peukert’s small correction for lithium is applied
Wire the same appliance directly to the battery (DC) and it runs about 10.5 hours, because no energy is lost in an inverter. The full method, including Peukert’s law for lead-acid batteries, is on how battery runtime is calculated.
How long will a 12V 100Ah battery last?
Runtime of a single 12 V 100 Ah battery at common loads, through a 90% efficient inverter. Lithium is used to 90%, AGM lead-acid to 50%.
| Load | LiFePO4 (90% usable) | AGM (50% usable) |
|---|---|---|
| 50 W | 19 h 26 min | 10 h 48 min |
| 100 W | 9 h 26 min | 4 h 55 min |
| 200 W | 4 h 33 min | 2 h 13 min |
| 300 W | 2 h 58 min | 1 h 24 min |
| 500 W | 1 h 44 min | 46 min |
| 1000 W | 50 min | 21 min |
The gap widens at higher loads: at 1,000 W the AGM battery is discharging at close to 1C, where Peukert’s law takes a large bite out of its capacity. For loads that big, lithium or a bigger bank is the practical choice. To go the other way and find the capacity you need for a set number of hours, use the battery size calculator.
What changes battery runtime
- Battery chemistry. LiFePO4 delivers most of its rated capacity at almost any load. Lead-acid gives less at high currents and should not be drained past about 50% for daily use.
- Inverter losses. Converting 12 V DC to 120 V or 230 V AC typically costs 8–15%, and an inverter draws power even with nothing plugged in.
- The real load. Fridges, pumps and air conditioners cycle on and off, so their average draw is well below their running watts. Use the average for runtime and the peak to size the inverter.
- Temperature. Cold batteries deliver less. Lead-acid can lose 20% or more near freezing, and most lithium batteries cannot be charged below 0 °C.
- Age. Every battery loses capacity over its life. A battery at 80% health runs 20% shorter; set this under Advanced settings.
Working on something specific? There are tuned versions of this calculator for UPS backup, home inverters, portable power stations, 12 V RV and boat batteries and small devices rated in mAh.