Battery runtime calculator

Estimate how long a battery will power a device, from the battery's capacity and the device's power draw.

Battery capacity (Wh)Device power draw (W)

Result

Estimated runtime

How to use this calculator

Find the battery's capacity in watt-hours (Wh) — if it's given in mAh, multiply by the voltage and divide by 1000 to get Wh.
Find the device's power draw in watts, from its label or spec sheet.
Read the estimated runtime.

Assumes the device draws a constant, steady power the whole time — real devices often vary their draw depending on load.
Doesn't account for battery efficiency losses or the reduced usable capacity of a battery as it ages.

Technical details: what the fields mean

Battery capacity batteryCapacityWh: Battery capacity in watt-hours. If your battery is rated in mAh (common for phone/tool batteries), Wh = mAh × voltage ÷ 1000.
Device power draw devicePowerWatts: The device's power draw in watts while running — check its label or spec sheet.

Methodology

Runtime (hours) = battery capacity (Wh) ÷ device power draw (W) — a direct division.

Worked example

100Wh battery, 20W device

Inputs: batteryCapacityWh=100, devicePowerWatts=20

Result: 5.00 hours

Practical guidance

For a phone or tool battery rated in mAh, convert to Wh first: watt-hours = (mAh ÷ 1000) × voltage.
Real-world runtime is often somewhat lower than this estimate, since batteries lose some capacity to internal resistance and age.

Common mistakes

Entering a battery's mAh rating directly as if it were Wh — these are different units and need the voltage to convert between them.

Multiply the mAh rating by the battery's voltage, then divide by 1000 — e.g. a 3000mAh, 3.7V battery is about 11.1Wh.

Divide its capacity in Wh by the device's power draw in watts — this calculator does that directly.