Wh vs mAh: reading battery capacity the right way
Why watt-hours compare batteries fairly, what milliamp-hours actually measure, and how voltage ties the two ratings together.
What each rating measures
A milliamp-hour rating counts charge: how many milliamps a cell can deliver for one hour. A watt-hour rating counts energy: power multiplied by time. The bridge between them is voltage, because watt-hours equal amp-hours multiplied by volts. That single relationship is why a phone bank and a power tool pack cannot be compared on milliamp-hours alone.
Why mAh alone can mislead
Marketing often leads with milliamp-hours because the number looks large. A 10,000 mAh power bank sounds bigger than a 3,000 mAh phone battery, yet the useful comparison is energy. If the bank stores its charge at 3.7 volts internally, that is 37 watt-hours, while a laptop battery at 11.1 volts and 4,000 mAh holds about 44 watt-hours. Convert both to watt-hours and the true size becomes clear.
Getting from one to the other
To go from amp-hours to watt-hours, multiply by the pack voltage. A 12 volt 100 Ah battery is 1200 watt-hours. To reverse it, divide watt-hours by voltage to recover amp-hours. Remember that milliamp-hours are just amp-hours times 1000, so a 2.6 Ah cell is 2600 mAh. The calculator on this page performs each of these steps as soon as you supply a voltage.
Why airlines and shippers care
Carry-on rules for lithium batteries are written in watt-hours, not milliamp-hours, precisely because watt-hours capture real energy. Most airlines allow spare batteries up to 100 Wh freely and up to 160 Wh with approval. Converting your pack rating to watt-hours before you fly tells you which bracket it falls into and whether you need to declare it.