The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter. The total capacity of a lead–acid battery is a function of the rate at which it is discharged.

Manufacturers supply run-time rating in minutes for packaged UPS systems. Larger systems (such as for data centres) require detailed calculation of the load, inverter efficiency, and battery characteristics to ensure the required endurance is attained.