Generator Ratings, Standby, Prime and Continuous

These ratings can lead to a level of confusion to many who do not live and breath generators on a daily basis as we do, this is also understandable as many manufacturers have slightly different definitions of their ratings and some have added in a few extra definitions to keep in touch with the changing market. For now, we will just stick to the typical main rating definitions to hopefully provide some clarity.

The generator rating is specified set of operating values provided by the particular manufacturer.  The generator has been designed to provide this value of power output under the different conditions or ratings dependent upon these operational modes.

These ratings are typically expressed in Kilowatt (kW)  and Kilo-volt-amperes (kVA)  at certain Voltage and Frequency outputs.  It’s also important to note that ambient conditions and altitude will also have an effect on the machines final rating.

Standby Power Rating

This is a rating for a generator set used for emergency or backup power for a facility (i.e. for use when the normal power supply fails, typically the utility). 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%-85%, (manufacturer specified) of the nameplate rating and applied to varying loads. This standby rating is its maximum possible output for short durations only, to be taken into account with the average load factor. Most manufacturers may place a limitation on the hours of operation per year, typically between 200-500hours.

Prime Power Rating

Prime power refers to generators used as the main source of electrical power (i.e. the power supply for an off-grid facility or utility parallel operation). Prime power generators are designed to operate for long periods of time at variable load. A typical prime power generator set may operate an unlimited number of hours per year, because of this, its power output will be less than for a standby generator set. This rating will allow up to a 10 percent overload for a specified number of hours per day, typically 1 hour in every 12. The rest of the time the set will operate under varying load conditions that are less than its rated power output.

Continuous Power Rating

The continuous power generator set may run at this rating for an unlimited number of hours per year, typically for base loading applications in parallel with the utility and co-generation applications.  The load is usually constant at 100 percent of the continuous rating. Given this mode of operation this is the most conservative rating.

For example, a particular sized generator package with the same engine & alternator combination may have these typical ratings

• Standby rated – 1,100kVA
• Prime power rated – 1,000kVA
• Continuous power rated – 910kVA

As mentioned some manufactures also publish alternative ratings such as Emergency Standby Power (ESP), Mission Critical Standby or Data centre rating, Limited Time Running Power (LTP) and these evolve over time, but the important point to take from this is that a particular generator is rated differently based on its actual application.