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Prime vs standby rating: what the numbers on a generator mean

The same generator set carries two or three numbers on its data sheet. Comparing quotes on the wrong one is the most common reason one offer looks 10% cheaper than another.

Standby (ESP)

The maximum power for emergency use when the grid fails. Typically limited to about 200 hours a year, with no overload allowed. Under ISO 8528 the average load over 24 hours should not exceed about 70% of the standby rating.

Prime (PRP)

Power available for an unlimited number of hours a year at variable load, with a 10% overload allowed for one hour in twelve. This is the rating to use when the generator is your main supply: mines, remote sites, rental, construction.

Continuous (COP)

Constant 100% load for unlimited hours, used mainly for base-load power plants. Less common in quotations.

How they relate

For most diesel sets, standby is about 10% above prime. A set sold as "1,100 kW" may be 1,000 kW prime. When you compare quotes:

• Ask which rating the headline number is.
• Compare prime with prime.
• Check that the engine model's own prime output supports the set rating; the alternator should be sized at or above it.

All prices we publish are quoted on the prime rating, with the standby figure shown alongside.