How many charges will a power bank give?
Power banks are rated in milliamp-hours (mAh) at the voltage of their lithium cells, usually 3.6 or 3.7 V. Phones list their battery in mAh too, at a slightly higher voltage. Comparing the two directly overstates the number of charges, because part of the energy is lost on the way. This calculator converts both to watt-hours and applies a conversion efficiency.
The formulas
- Power bank energy (Wh) = mAh × cell voltage ÷ 1,000
- Usable energy = Wh × efficiency
- Device battery (Wh) = device mAh × device voltage ÷ 1,000
- Full charges = usable energy ÷ device battery
- Time to recharge the bank ≈ Wh ÷ (charger watts × 85%)
Worked example
A 10,000 mAh power bank and a phone with a 3,500 mAh battery at 3.85 V:
- Power bank: 10,000 × 3.7 ÷ 1,000 = 37 Wh.
- Usable at 70% efficiency: 25.9 Wh.
- Phone battery: 3,500 × 3.85 ÷ 1,000 = 13.5 Wh.
- Charges: 25.9 ÷ 13.5 = 1.9 full charges.
- With an 18 W charger, the bank refills in about 2 h 25 min.
Where the energy goes
The cells deliver 3.7 V, but USB runs at 5 V or more, so the bank boosts the voltage; the phone then steps it back down to charge its own battery. Each conversion, plus heat and cable resistance, costs energy. Real-world efficiency ranges from about 60% for cheap banks and fast charging to 80% or more for good ones.
Common capacities
| Power bank | Energy | Typical phone charges |
|---|---|---|
| 5,000 mAh | 18.5 Wh | about 1 |
| 10,000 mAh | 37 Wh | about 2 |
| 20,000 mAh | 74 Wh | about 3.5 to 4 |
| 26,800 mAh | 99 Wh | about 5 |
Flying with a power bank
Spare lithium batteries, power banks included, must travel in carry-on baggage, never in checked bags. Up to 100 Wh they are allowed without approval; between 100 and 160 Wh, airlines usually allow two with their approval; above 160 Wh they are forbidden on passenger flights. At 3.7 V, 100 Wh is about 27,000 mAh, which is why many large banks stop just under that size. The Wh figure is usually printed on the label.