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A power bank labelled 20,000mAh does not necessarily deliver 20,000mAh at the USB voltage your device uses. The headline commonly describes capacity at the internal cells’ nominal voltage. Watt-hours make the energy comparison clearer.
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Convert using the stated voltage
The calculation is watt-hours = milliamp-hours × volts ÷ 1,000. You need the voltage associated with the capacity claim, not a convenient voltage copied from a USB port.
For a hypothetical battery rated 20,000mAh at 3.7V, the result is 74Wh. At a hypothetical 5V output, 74Wh would equal 14,800mAh before conversion losses. No energy has disappeared in the arithmetic; the voltage basis has changed.
Prefer a printed Wh figure
If the manufacturer provides watt-hours, use that figure rather than assuming every battery has the same internal voltage. For example, Anker specifies its A1695 25,000mAh model at 90Wh. Multiplying by an assumed 3.7V would produce a different result.
Check the label and documentation for the exact model. A similar-looking power bank with the same headline mAh may use a different capacity basis or have different output limits.
Allow for the charging chain
The battery’s nominal stored energy is not identical to energy delivered into your phone’s battery. Voltage conversion, cable losses and the phone’s own operation all affect the result. Temperature, battery condition and charging settings can also change practical performance.
For an illustration, suppose a bank stores 90Wh and you assume 75% reaches a device battery after the whole charging process. That gives 67.5Wh. If the device battery were 15Wh, the arithmetic suggests 4.5 full-equivalent charges. Both figures are assumptions, not a measured claim for the Anker model or your phone.
Do not confuse capacity with speed
Watts describe power: how quickly compatible equipment can transfer energy at a given moment. Watt-hours describe an amount of energy. A 100W-capable output does not mean the power bank stores 100Wh, and a phone will not automatically take 100W.
The Anker 25K 165W model is one example where the product name includes both a capacity and a power figure. Read them separately when comparing alternatives.
Make a trip-specific estimate
Record how much charge you use on a representative day and test the power bank with your actual phone and cable. Navigation, photography and poor reception can make camping use differ from a quiet day at home. Plan from that experience, with a reserve, rather than relying on a universal number of phone charges.
Put the conversion into a trip plan with our seven-day power bank budget. For charging speed and cable compatibility, read the separate USB-C camping power bank guide.
Continue with our camping power bank guide.
Reviewed 10 September 2026. This guide is based on published specifications and guidance.