# Understanding Generator Set Ratings

Generator set ratings may seem complex, but their basic purpose is simple: fit the application needs at the optimum reliability, performance, and cost. An improper rating means either buying more capacity than needed or risking shorter life to overhaul, more repairs, and more downtime.

Ratings have changed in recent years, and more sophisticated switchgear can be integrated with generator sets. That means more flexibility to specify generator set systems that closely match a specific installation requirement. 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 generator sets run?
- Will the generator sets be run isolated or in parallel with the utility?

### Ratings Defined

Caterpillar defines five basic generator set ratings:

- Standby
- Emergency Standby Power (ESP)
- Mission Critical Standby
- Prime
- Continuous

### 1. 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 Standby rated generator set should be **no more than 70%** of the nameplate rating and applied to varying loads. A Standby generator set can run for a **maximum of 500 hours per year.** **No overload is available.** The normal standby rating is not for use in utility paralleling applications.

For example, a 3 MW standby rated 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% average load factor with varying load**. An example of the Standby and ESP ratings are shown below.

*Figure 1: Example Load Profile - 3 MW Standby*

### 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% of the nameplate rating** with varying loads. A mission critical standby generator set typicall runs about 200 hours a year, with a **maximum of 500 hours per year**. No overload available. Typical peak demand is 100% of the rating for **maximum of 5% of the operating time**. The mission critical standby rating is not for use in utility paralleling applications.

An example of the mission critical standby rating is shown in Figure 2. For example, a 3 MW mission critical standby-rated generator set will provide power for the duration of an outage. It could be run for up to 500 hours per year and have an average load factor of up to 2.55 MW.

*Figure 2: Example Load Profile: 3MW Mission Critical Standby Rating*

### 4. Prime

In this application, the generator set is capable of providing power to a** varying load for an unlimited number of hours per year**. A Prime rated generator set is capable of providing full nameplate rating for a period of time, but must have an average load factor of no more than **70% of the Prime rating**. **10% overload is allowed** for emergencies for a **maximum of 1 hour in 12 hours**, and for **no more than 25 hours per year**. Time spent during operation above 70% load may affect the life to overhaul of the generator set.

The standard prime rating is for use in either utility paralleling or isolated applications. For example, a 2.7 MW rated unit may provide the full nameplate rating for a short duration, but should have a maximum average load of 1.89 MW (not including generator set non-running time per ISO8528-1). The generator set can also provide 3 MW of power in emergencies as defined above. An example of the Prime rating is shown in Figure 3.

*Figure 3: Example Load Profile: 2.7 MW Prime Rating*

### 5. Continuous

In this application, the generator set is able to provide power to a **non-varying load for an unlimited number of hours per year.** The average power output of the generator set is **70 -100% of the rating**. The rating is designed to provide 100% of the rating for 100% of the operating hours. Typical Continuous rating applications include base loading in parallel with the utility and co-generation operations. An example of a continuous rated generator set is shown in Figure 4.

*Figure 4: Example Load Profile: 2.5 MW Continuous Rating*

Regardless of the application, generator set ratings help ensure that your power needs are met and that generator set is protected from premature wear. Choosing the right rating means making the proper tradeoffs between run hours, peak load, and average load. The proper rating means you receive the optimum combination of installed cost and long-term cost of ownership.

Read how Cat generator set ratings compare against those defined by the industry standard ISO8528-1:2018 here.

See our full range of generator sets here.