Free browser tool
Battery runtime calculator
Enter the battery capacity in watt-hours and the load in watts. Add conversion efficiency, a reserve you never want to use and any standby overhead, and get the runtime with every intermediate value shown exactly.
Runs on this device. No upload, account or tracking.
Battery and load
Usually printed on the battery as Wh. Examples use typical values; check your own label.
Only know mAh? Convert to Wh
Wh = mAh × V ÷ 1,000. Phone and laptop cells are usually rated at 3.6–3.85 V.
The average power the device draws while running.
Inverter or converter efficiency, above 0 and at most 100%. 85% is a common inverter assumption.
Share of nominal capacity you never want to use, from 0 to below 100%.
Advanced: standby overhead
A constant extra draw, such as an inverter's idle consumption. Each value: at most 64 characters and 28 significant digits; non-zero values from 0.000000001 to 10^15.
The result updates as you type. Nothing leaves this page.
Your runtime appears here
Enter a capacity and a load, or try an example.
Estimated runtime
Technical report (JSON)
The download contains the same report.
About this tool
What this tool does
Estimate how many hours a battery or power bank runs a load, from its watt-hour capacity, the load in watts, inverter or converter efficiency and a reserve you keep back.
Limits and safe use
An estimate from the numbers you enter: real runtime also depends on temperature, battery age, discharge rate and the load's actual draw. Review the assumptions before relying on the estimate. It is a deterministic calculation, not a measurement or safety advice, and it does not send data to a server or a model.