Power Bank Charge Calculator

How it works

power bank Wh = mAh × cell V ÷ 1,000 · usable Wh = Wh × efficiency · phone Wh = phone mAh × phone V ÷ 1,000 · full charges = usable Wh ÷ phone Wh · top-ups = usable Wh ÷ (phone Wh × (100 − start%) ÷ 100)

A power bank’s mAh rating refers to its internal lithium cells at about 3.7 V, but it delivers power at 5 V or more through USB, and the phone then converts it back down to charge its own battery. Each step loses energy as heat, so only about 60–80% of the rated energy reaches the phone. Comparing watt-hours rather than mAh removes the voltage confusion: that is why a 10,000 mAh power bank charges a 4,500 mAh phone about 1.5 times, not 2.2 times.

Worked example

10,000 mAh × 3.7 V = 37.0 Wh rated; × 70% = 25.9 Wh usable. The phone holds 4,500 mAh × 3.85 V = 17.33 Wh, so 25.9 ÷ 17.33 = 1.5 full charges — or 1.9 top-ups from 20% to 100% (each needing 13.86 Wh). The usable energy equals 6,727 mAh at the phone’s voltage.

Sources: FAA PackSafe — Lithium batteries · Battery University — BU-105 Battery definitions

Frequently asked questions