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Tool

Power bank calculator

Roughly how long could your power bank keep a phone, light or router going in a power cut? Enter what is printed on the battery and the device.

Your power bank’s capacity is printed as
For example 37 Wh, or 10000 mAh
Look on the label of the device or its power adapter.
We assume 85% by default
To estimate full charges. Check your phone or device specifications.

How we work it out

  • If you enter mAh, we convert to watt-hours: mAh × voltage ÷ 1000. Power banks are usually rated at the internal cell voltage of 3.7 V.
  • Some energy is lost converting the voltage and as heat. We assume 85% reaches your device. That is an assumption, not a measured figure for your power bank; change it if you know better.
  • Run time = usable watt-hours ÷ device watts. Devices rarely draw a steady wattage, so treat the answer as a rough guide.
Battery safety

Lithium batteries can overheat or catch fire if damaged or charged badly. Buy from reputable sellers, look for UKCA or CE marking, use a suitable charger, don’t charge on a bed or sofa, and stop using any battery that is swollen, damaged or unusually hot. If you rely on powered medical equipment, ask the team who supplied it about backup power, and see our Priority Services Register guide.