Watt-Hour Calculator
Convert battery volts and amp-hours to watt-hours and kilowatt-hours, and estimate how long a load will run.
Battery energy
1,200.0 watt-hours
- Kilowatt-hours
- 1.200 kWh
- Usable energy
- 1,200.0 Wh
- Estimated runtime
- 12:00:00
The battery stores 1,200 Wh (1.2 kWh). With 1,200 Wh usable, a 100 W load would run for about 12 hours.
Calculation breakdown
| Step | Formula | Result |
|---|---|---|
| Battery energy | 12 V × 100 Ah | 1,200.0 Wh |
| Usable energy | 1,200 Wh × 100% × 100% | 1,200.0 Wh |
| Runtime | 1,200 Wh ÷ 100 W = hours | 12:00:00 |
How it works
Enter the battery’s voltage and capacity. Add a load in watts to see how long it will run, and open the advanced options to allow for depth of discharge and inverter efficiency.
Worked example
A 12 V, 100 Ah battery powering a 100 W load, with 80% usable depth of discharge and a 90% efficient inverter:
- Battery energy: 12 × 100 = 1,200 Wh (1.2 kWh)
- Usable energy: 1,200 × 0.80 × 0.90 = 864 Wh
- Runtime: 864 ÷ 100 = 8.64 hours, which is 8:38:24
Rounding
Energy is shown to one decimal place. Runtime is shown as hours, minutes and seconds.
Sources
Frequently asked questions
How do you calculate watt-hours?
Multiply the battery voltage by its capacity in amp-hours. A 12 volt battery rated at 100 amp-hours stores 12 × 100 = 1,200 watt-hours. Divide by 1,000 to get kilowatt-hours, which is 1.2 kWh.
How do I convert amp-hours to watt-hours?
Multiply the amp-hours by the voltage. Without the voltage, amp-hours alone cannot be converted, because the same amp-hour rating holds more energy in a higher-voltage battery. A 3.7 volt, 5 amp-hour (5,000 mAh) phone battery holds 18.5 watt-hours.
How long will a battery run a device?
Divide the usable watt-hours by the load in watts. 1,200 watt-hours powering a 100 watt load lasts 12 hours. Add the depth of discharge and inverter efficiency in the advanced options for a more realistic figure.
What is depth of discharge?
It is the share of a battery's capacity you plan to use. Many batteries last longer if they are not drained completely, so a maker may advise a limit such as using only part of the rated capacity. Enter the figure from your battery's documentation.
Why is the runtime less than the calculation?
Real batteries lose some energy as heat, inverters waste a share of the power they convert, and capacity changes with load, age and temperature. Enter an inverter efficiency below 100% to account for conversion losses. Treat the result as an estimate.
Estimates only. Real battery capacity varies with load, temperature and age, so check your battery maker's data.