Prime Rating generators are the one intended to run without any restriction of running hour. Output of these generators can be of varying load to a peak demand of 100% for an unlimited time. A 10% of overload capability is allowed for emergency use for a maximum of 1 hour in 12 hours.

Standby Rating generators are the ones for supplying emergency power for the duration of normal power interruption. No sustained overload capability is permitted for this rating.