Genset Rating Classifications

ISO 8528 identifies four different generator rating classifications: continuous, prime, limited-time power, and emergency standby. Many companies also  employ two additional classifications to further define the ratings for its generator products.

Continuous – As the term implies, continuous power generators are designed to provide power on an ongoing basis throughout the year. The average output of a continuous power gen set is 70 to 100 percent of the rating and is designed to provide 100-percent power for every operating hour during the year. Continuous power gensets are best used in situations where a limited amount of power load fluctuation occurs.

Prime – Prime gen sets also provide ongoing power, but unlike continuous generators that are designed for limited load fluctuations, prime generators can accommodate varying loads on an unlimited basis throughout the year. However, the average load factor cannot exceed 70 percent of the prime rating.

Limited-Time – Gen sets with a limited-time power rating are designed to operate at a maximum of 500 hours per year, although they can effectively manage an average load factor of up to 100 percent.

Mission Critical Standby – This specific rating is intended to comply with the higher standby power requirements that apply to entities such as data centers. These types of generators provide an average load factor of 85 percent, as opposed to the 70 percent offered by ESP generators.

Emergency Standby – ESP generators are designed to provide a short-term power solution when an unexpected loss of a continuous or prime power source occurs. ESP-rated generators are generally intended to operate at a maximum of 200 hours per year at an average load factor of 70 percent. Additionally, the average power output should not exceed 70 percent in any 24-hour period.

Standby – standby gensets differ from ESP units in that they are designed to provide emergency power for the duration of an outage. The average load factor of these gen sets is 70 percent, with a maximum operating time of 500 hours per year.