# Battery Runtime Calculator

> Battery runtime calculator: estimate how long a battery lasts from capacity, voltage, load watts, and efficiency. Free, private, runs in your browser.

- **Canonical URL:** https://boringtoolskit.com/battery-runtime-calculator/
- **Category:** Energy & Electrical
- **Evidence level:** authored-methodology
- **Reviewed:** July 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Nominal energy (Wh) = amp-hours × volts; runtime = nominal energy × usable fraction × efficiency ÷ load watts.

## Assumptions

- Voltage and load remain constant.
- Efficiency and usable capacity are user-entered aggregate factors.
- Peukert effects, temperature, degradation, surge loads, and reserve margins are excluded.

## Common questions

### How do I calculate battery runtime?

Divide the battery's energy in watt-hours by the device's power in watts, then multiply by your system efficiency and usable capacity. Energy is capacity (Ah) × voltage (V), or the Wh rating directly. This calculator does the unit conversions for you.

### What is the difference between mAh, Ah, and Wh?

mAh and Ah measure charge (1 Ah = 1,000 mAh); Wh measures energy. Wh = Ah × voltage, so a 3,000 mAh battery at 3.7 V holds about 11.1 Wh. Use Wh when comparing batteries with different voltages.

### How do I convert amps to watts?

Watts = volts × amps. A device drawing 2 A at 12 V uses 24 W. If you know the load in amps or milliamps, this calculator converts it to watts using your battery voltage.

### Why does my actual battery life differ from the calculation?

Real batteries lose energy to heat, internal resistance, voltage regulation, and discharge rate (Peukert effect), and they age and run less efficiently in cold. The efficiency and usable-capacity inputs give a more realistic estimate than the theoretical number.

## Related tools

- /watts-to-amps-calculator/
- /electricity-cost-calculator/
- /ev-charging-cost-calculator/

## Sources

- [Energy storage basics](https://www.energy.gov/oe/energy-storage)
- [Energy storage systems](https://www.sandia.gov/ess-ssl/)

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