mAh vs Wh (battery capacity)

A battery's mAh rating says how much current it can supply for how long but says nothing about voltage. A 3,000 mAh single-cell pack at 3.7 V stores about 11 Wh; a 3,000 mAh four-cell series pack at 14.8 V stores about 44 Wh. Watt-hours compare energy directly and are what power budgets, shipping regulations and airline rules use. In the US, spare lithium-ion batteries of 101–160 Wh need airline approval to be carried by passengers, and larger ones are generally not allowed in passenger baggage.

Capacity ratings on power banks are usually given at the internal cell voltage, so the energy actually delivered to a device at a higher output voltage, after conversion losses, is lower than the headline mAh suggests.