Rated vs usable capacity
A power station's headline number is the energy stored in its cells, in watt-hours. The energy that reaches your devices is always less, for three reasons.
Inverter losses
Converting DC battery power to 120V AC costs energy. Good inverters are 85–92% efficient at moderate loads; efficiency falls at very light loads, where the inverter's own overhead dominates, and at loads near the maximum.
Reserve capacity
Most units stop discharging at a few percent state of charge to protect the cells, and some reserve more.
Self-consumption
The unit's electronics, display and fan draw power while it runs. At a 20W load, a 15W idle draw nearly halves your runtime.
What independent tests show
Reviewers who time a full discharge into a known load typically report 75–90% of rated capacity delivered through the AC outlets, and somewhat more through USB-C. The exact figure depends on the load used, which is why we record the load alongside every measurement we log.
How we use it
When a unit has an independent usable-capacity measurement, it becomes a scoring component in our formula. Until then the unit is ranked on specifications and labelled "spec-based." See the reviewers we track.