The 5 Basic Generator Set Ratings

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Generator set ratings may seem complex, but their basic purpose is simple: fit the customer’s needs at the optimum reliability, performance, and cost levels.

An improper rating means you are either buying more capacity than needed or risking shorter life to overhaul, more repairs, and more downtime.

The key to choosing the right rating is to understand the application in detail. That means not only knowing the type of duty but also answering:

  • What is the average load factor?
  • What is the maximum required load?
  • How many hours per year will the generators sets run?
  • Will the generator sets be run isolated from or parallel with the utility?

Here are basic descriptions of the genset ratings:

  1. Standby – The average load factor of a standby generator set should be no more than 70 percent of the nameplate rating when applied to varying loads. A standby generator set can run for a maximum of 500 hours per year. The normal standby rating is not for use in utility paralleling applications.

    For example, a 3MW standby generator set will provide power for the duration of an outage. It should be run for up to 500 hours per year and have an average load factor of 2.1 MW.

  2. Emergency Standby Power (ESP) – The ESP rating differs from the standby rating only in the number of running hours allowed per year. ESP ratings allow a maximum running time of 200 hours per year at a 70 percent average load factor with varying loads.

  3. Mission Critical Standby – 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 mission critical standby rated generator set should be no more than 85 percent of the nameplate rating with varying loads. A mission critical standby generator set can run for a maximum of 500 hours per year.

  4. Prime Power – Intended for high-hour operation involving remote installations, peak shaving applications, rental and co-generation with varying loads.

  5. Continuous – This is for operating at 100 percent of the nameplate rating for extended periods of time in base load or operating as the prime power source at remote sites.

The gensets are built using robust engines that incorporate advanced engineering, design, and manufacturing techniques to create a product that product that preforms in the world’s most rugged conditions. Genset ratings can be optimized for specific applications if a deep understanding of the loads, load schemes, and application specifics are taken into account.

We offer an extensive engineering, application, and design knowledge base to assist you in selecting, configuring and optimizing your equipment. Contact our power systems experts for more information about matching genset reliability, performance and cost parameters to your application requirements.