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Battery run time calculator
How long will a battery or portable power station run a light, fan, router or CPAP machine?
How it is worked out
Run time (hours) = battery energy (Wh) × efficiency ÷ device power (W). If the capacity is in mAh, energy in Wh = mAh × volts ÷ 1,000 — for a power bank or phone battery, use the cell voltage printed on it (usually 3.6 to 3.85 V). Some energy is always lost turning the battery's power into what the device needs, especially through an AC inverter; 85% is a typical round figure, but use your device's rating if it gives one.
Getting more hours
- Power devices directly from DC (USB or 12 V) rather than through an AC inverter when you can.
- Keep batteries warm: lithium batteries give much less in freezing weather.
- LED lights use a fraction of the power of old bulbs.
Charging phones? See the power bank calculator.
For planning and general information. In an emergency, follow the instructions of your local authorities and call your emergency number (911, 112, 999 or 000).
More power and energy tools
Generator wattage calculatorWhat size generator you needGenerator run time and fuel calculatorHours on a tank of fuelHow many phone charges in a power bank?Phone charges per power bankSolar panel output calculatorWatt-hours a day from a panel
Source: arithmetic (energy = capacity × voltage).