Prime Power vs Standby Power

The same generator can carry different ratings depending on how it will be used.

Prime and standby ratings describe different operating duties. The correct rating must be considered together with expected load, annual operating hours and the applicable engine and generator manufacturer's rating conditions.

1. Prime Power

Prime Power is used where the generator is expected to operate for extended periods as a primary or regularly used power source. Under the example rating below, the generator can operate at its 300 kVA Prime rating, while the average load should be 70% of that rating: 210 kVA.

2. Prime Power operating hours and overload

For this example, annual Prime Power running hours are not limited. Subject to the manufacturer's applicable guidelines and hour limitations, a 10% overload may be available for 1 hour in every 12 hours of operation. On a 300 kVA Prime-rated generator, that gives an example overload rating of 330 kVA.

3. Standby Power

Standby Power is intended for emergency backup operation when the normal mains supply fails. Using the same example generator, the Standby rating is 330 kVA and the average load should be 70% of that rating: 231 kVA.

4. No overload at the Standby rating

There is no additional overload available above the Standby rating in this example. Manufacturer restrictions on permitted Standby operating hours must also be taken into account.

5. Why the application must be specified

A customer needing emergency backup during mains failures has a different operating profile from a site using a generator as its regular source of power. Telling GenTech how the generator will be used helps us select and quote the appropriate rating and configuration.

6. Manufacturer ratings take priority

The 300/330 kVA figures above are an explanatory example. Prime, standby, overload and operating-hour conditions must always be checked against the applicable engine and generator manufacturer's technical data for the specific generator being supplied.