Battery runtime calculator
Estimate how long a battery will power a device, from the battery's capacity and the device's power draw.
Result
Estimated runtime
How to use this tool
- 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.
Assumptions
- 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: Battery capacity: 100 Wh, Device power draw: 20 W
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.
FAQs
How do I convert mAh to Wh?
Multiply the mAh rating by the battery's voltage, then divide by 1000 — e.g. a 3000mAh, 3.7V battery is about 11.1Wh.
How long will my battery last?
Divide its capacity in Wh by the device's power draw in watts — this calculator does that directly.
Last reviewed: 2026-09-25
Assumes a constant power draw and ignores battery efficiency losses and age-related capacity fade — real-world runtime is often somewhat lower than this estimate.
This tool provides an indicative estimate, based on the assumptions stated above.
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