Most generator sets are sold for “backup” power, to supply electricity when the utility power has failed. These are rated as “standby” generators, and while rated as “Continuous” duty – meaning they will not overheat if used continuously – this is an “emergency” rating, not a rating for long life of the generator end or engine.

“Prime Power” is the rating that should be used when a generator is to supply power in place of utility power, or in cases where it will supply power for significant portions of time on a regular basis when utility power is less than reliable.

We recommend purchasing generator sets based on the Prime Power ratings, even when they will be used for standby power in developing countries. This will result in longer life and better return on the investment.

The prime power gen-set engine rating is the nominal power an engine is capable of delivering with a variable load for an unlimited number of hours per year with normal maintenance intervals observed. This rating incorporates a 10% overload capability which is available for up to 2 hours at a time. Operating time between 100 % and 110% of the prime power rating is not to exceed 8% of the total engine operating time. This rating conforms to ISO 8528-1 “prime power (PRP)”. The permissible average power for the prime or PRP rating is not to exceed 70% of rated prime power when calculated per ISO 8528-1.

The standby gen-set engine rating is the nominal engine power available at varying load factors for up to 200 hours per year with normal maintenance intervals observed. No overload capability is available for this rating. This rating conforms to ISO 8528-1 “Emergency Standby Power (ESP)”. The permissible average power for the standby or ESP rating is calculated per ISO 8528-1.