Battery Run Time Calculator
This calculator shows how long a battery or portable power station can run a device. Enter the capacity in watt-hours, or in amp-hours and volts, and the watts the device draws.
Run time on battery
Devices with a motor or compressor, such as fridges, cycle on and off, so their average draw is lower than the watts on the label. Use the daily watt-hours from the manual or the energy label where it is given.
How the calculator works
1. Capacity in watt-hours. Power stations list Wh. For a battery listed in amp-hours, Wh = Ah × volts, so a 100 Ah battery at 12.8 V holds 1,280 Wh.
2. Usable energy. Capacity × the usable share of the battery × inverter efficiency. The usable share is the depth of discharge the maker states. The inverter turns battery power into AC for devices that plug into an outlet and loses some as heat. For DC or USB devices set the efficiency to 100.
3. Run time. Usable energy ÷ device watts. A 1,000 Wh power station at 80% usable and 90% inverter efficiency gives 720 Wh, which runs a 60 watt device for 12 hours.
Where the starting values come from
| Input | Starting value | Where it comes from |
|---|---|---|
| Usable share of the battery | 80% | A placeholder. Use the depth of discharge on your battery or power station listing |
| Inverter efficiency | 90% | A placeholder. Use the efficiency on your inverter or power station listing |
Both figures change from one product to the next, so replace them with the numbers on your own listing.
Reading the result
- Label watts are the maximum. Many devices draw less than their label most of the time. A plug-in watt meter shows the real figure.
- Fridges and other compressors cycle. They run for part of each hour, so use the daily watt-hours from the manual or energy label and divide the usable energy by that to get days.
- Check the output rating too. Capacity sets how long, and the inverter’s continuous watts set whether the device runs at all. Motors also need a higher surge rating to start.
- Several devices. Add their watts together, or list them with their hours in the solar generator and battery size calculator.