Estimate only. Check the device's real power with a plug-in meter if you can: the label shows the maximum.
How the runtime is calculated
- Usable energy: capacity in Wh × output efficiency.
- Runtime: usable energy ÷ the device's watts.
Which efficiency to use
Power from the AC outlets goes through the power station's inverter, which loses part of the energy as heat. The default 85% is an assumption; your power station's manual or spec sheet may list its own figure. USB and 12 V outputs usually lose less.
The device must also stay under the power station's maximum output, or it will shut off. Devices with motors or compressors, such as refrigerators, briefly draw several times their running power when they start.
For medical devices such as a CPAP, follow your doctor's and the manufacturer's advice for backup power.