Understanding Generator Set Ratings

Generator set ratings are operating values that manufacturers provide for users, telling them how much power an engine can produce under certain conditions. These ratings are usually found on a metal plate affixed to the generator. 

 The International Standards Organization, an independent non-governmental organization, develops the recommended generator ratings for manufacturers to use. The standards listed under ISO-8528-1 apply to reciprocating internal combustion (RIC) engines used in generators for land and marine applications, with the exception of aircraft, land vehicle propulsion, and locomotives. 

 Several factors are considered when setting generator ratings, including: 

•     The engine’s ability to manage the electrical load
•     Duration of the load
•     Expected life cycle of the generator
•     Mean time between overhauls (how long the engine can operate under average conditions before maintenance)
•     Ambient conditions (such as climate and altitude)
•     Fuel
•     Manufacturer’s maintenance program

Generator ratings are useful tools to help customers select the right equipment for their needs. They’re also important guidelines for proper use of the equipment. Using a generator beyond its rating capacity can have serious consequences, such as system failure, voiding of the manufacturer’s warranty, and longer and pricier repairs. By the same token, using a generator according to its rating capacity can result in a longer lifespan for the equipment and a lower overall cost of ownership for the customer.    

Here is a breakdown of the most common ISO generator set ratings.

Emergency Standby Power (ESP)

This generator rating is normally used for equipment that is connected to a commercial power utility but used to provide emergency power when that utility suffers an outage. The rating represents the maximum amount of power a generator set can deliver. The average load factor is set by the manufacturer but is usually 70% of full capacity, and there is no overload capability. ESP-rated generators may run for a maximum of 200 hours per year under Environmental Protection Agency emissions regulations.

Prime Power (PRP)

This generator set rating is normally applied to generators used without a connection to a commercial power utility. Typical applications include temporary use, load management, or to power off-grid locations such as wilderness outposts and remote mining, quarrying, or drilling operations. The average load factor is usually 70% of full capacity, but unlike an ESP-rated generator, a PRP-rated engine can run for an unlimited number of hours. A 10% overload capacity is permissible for short periods of time (a maximum of one hour per 12 hours, and a cap of 400 hours per year).

Limited-Time Running Prime

This generator rating is for equipment that provides constant power loads for short periods of time. These generators are often used to supply power to a utility as part of an agreement for purposes like peak shaving, load curtailment, and cogeneration. The average load factor can be 100% for a limited number of hours, an amount determined by the manufacturer. Should additional power be needed, the continuous power rating outlined next applies.

Continuous Power (COP)

Also known as “base-load rating,” the COP rating applies to generators that provide constant power loads for long periods of time. Typical applications of these generators include utility base-load generation and cogeneration, or when the equipment is the main power source for a remote site that commercial utilities cannot reach. The average load factor for these generators is 100% for an unlimited number of hours, but there is no overload capability for a COP-rated generator.

Data Center Continuous Power (DCP)

This is a relatively new rating that applies to generators that provide power to data centers. Data centers need dedicated sources to deliver uninterrupted power, so DCP-rated generators can operate for an unlimited number of hours without restrictions on average load factor. These ratings are approved by the Uptime Institute, an independent advisory organization that oversees digital infrastructure worldwide.