Why 10,000 mAh is never 10,000 mAh
Contents4
Your phone has a 4000 mAh battery, your new power bank says 10,000 mAh. Two and a half charges, then? In practice it is barely two. Understanding why means buying the right size next time instead of being annoyed by the readout.
mAh alone says nothing
Milliamp-hours are only half a figure. They describe an amount of charge, but without a voltage that is like a distance without a unit. The cells in a power bank run at around 3.7 volts. The USB output, however, delivers 5 volts, or 9 to 20 when fast charging. The electronics have to step the voltage up, and energy is lost as heat along the way.
The honest unit is the watt-hour. Good models print it in small type on the housing: capacity in mAh divided by 1000, times 3.7 volts gives watt-hours. So 10,000 mAh becomes 37 Wh.
The rule of thumb: times 0.6
Convert the watt-hours to the 5 volts at the USB output and subtract conversion losses, and roughly 60 percent of the printed figure remains. As an everyday estimate that is plenty.
| Printed | Watt-hours | Actually usable | iPhone 16 charges |
|---|---|---|---|
| 5000 mAh | 18.5 Wh | around 3100 mAh | just under one |
| 10,000 mAh | 37 Wh | around 6300 mAh | about 1.7 |
| 20,000 mAh | 74 Wh | around 12,600 mAh | about 3.5 |
| 27,000 mAh | 100 Wh | around 17,000 mAh | the limit for air travel |
Where the energy goes
Part is lost stepping the voltage up, more in the phone's own charging circuit. Wireless adds a much bigger loss on top, which is why the same power bank delivers noticeably less when snapped on magnetically than over a cable. Cold costs extra: at five degrees in a jacket pocket the usable amount drops measurably, as described in phones in winter.
The number that matters when buying
Compare watt-hours, not mAh. If a model advertises 20,000 mAh but states only 60 Wh, the arithmetic does not add up: 20,000 mAh should be 74 Wh. The cells inside are then closer to 16,000 mAh. That mismatch is the most reliable sign of an inflated figure.
For a commute, 5000 mAh is enough. Planning a weekend away from sockets, or feeding a tablet as well, calls for 20,000. Which form factor suits is covered in which power bank do I need, and the models are in power banks.