Understanding Generator Size Ratings - Before You Load Test

Understanding Generator Set Ratings

Emergency Standby Rating (ESP)

The maximum power available during a variable electrical power sequence, under the stated operating conditions, for which a generating set is capable of delivering in the event of a utility power outage or under test conditions for up to 200 hours per year of operation with maintenance intervals and procedures being carried out as prescripted by the manufactures.  The permissible average power output over 24 hours of operation shall not exceed 70% of the ESP rating.  


In this application, the generator set is capable of providing emergency backup power at the nameplate rating for the duration of an outage. The average load factor of a Standby rated generator set should be no more than 70% of the nameplate rating and applied to varying loads. A Standby generator set can run for a maximum of 500 hours per year

Load Testing

Standby Generators are designed to work with variable loads.  If load bank testing is necessary, the consistent power shall not exceed 70% When a generator is properly sized, reserve power is available for variable loads such as motors, compressors and other devices that require more power for short periods of time in order to start up. Exceeding 70% on a continuous basis is considered overload. Overload results in overheating and may result in premature failure.