Watt-hour (Wh)

Watt-hours measure the total energy stored in a battery, and it is the number to compare when judging how long a portable power station or battery pack can run something, rather than amp-hours, which only make sense at a fixed voltage. Two stations rated at the same watt-hours store the same usable energy even if their battery designs differ.

To estimate runtime, divide the watt-hour rating by the wattage of the device you want to run, then reduce that result by roughly 10 to 15 percent to account for conversion losses in the built-in inverter and battery inefficiency. A 40-watt fridge on a 500Wh station gives a rough runtime of around 10 to 11 hours in real conditions, not the full 12.5 hours simple division suggests.

A common mistake is comparing two power stations purely by price per watt-hour without checking the inverter's continuous wattage, since a cheaper high-capacity unit that cannot handle your device's running or surge wattage is not actually usable for that job. Always check runtime against your specific gear rather than trusting a generic runtime claim on the box.