Battery & Power Bank Runtime Calculator

In short Milliamp-hours are not a measure of energy, which is why a 20,000 mAh power bank does not give four full charges to a 5,000 mAh phone. The missing term is voltage: watt-hours equal mAh times volts divided by 1000, and a power bank's cells run at about 3.7 V while it outputs at 5 V or higher. Converting up costs energy, and the conversion in the bank plus the conversion in the phone plus heat typically loses 30 to 40% of the nominal figure. Two and a half charges from that 20,000 mAh bank is a realistic expectation, and a bank advertising four is quoting cell capacity rather than delivered capacity. Comparing banks on watt-hours rather than mAh is the fix, and it is also the number airlines use: the common limit is 100 Wh in hand luggage without approval, which is roughly a 27,000 mAh bank at 3.7 V — larger ones need airline permission and are generally refused above 160 Wh. For runtime of a device rather than charges, divide the watt-hours by the device's average draw in watts, and remember that peak draw and average draw are very different numbers for anything with a screen.