Understanding how genset load factor affects mission-critical power

Understanding how average load factor can affect mission-critical standby power systems during emergencies can ensure continuity of electrical power—and business operations.

By Brandon Kraemer, MTU Onsite Energy, Mankato, Minn. November 10, 2011

Consider this nightmarish scenario: At midday, the sky turns the color of ink, sudden rain squalls flood neighborhood streets, and disaster sirens wail as an F5 tornado scores a direct hit on a power substation. Nearby, a national bank data center’s UPS system and emergency power generators kick in—as designed—to keep mission-critical operations online. But with the utility outage likely to last for days, facility operators ask themselves, “Will the standby generators be able to supply all power needs for the duration of the outage?”

The answer to that question is related in part to load factor. An important consideration in sizing gensets is calculating the application’s average load factor.

Average load factor

The average load factor of a power system is determined by evaluating the amount of load and the amount of time the genset is operating at that load. Since the loads are normally variable, the result is found by calculating multiple load levels and time periods. Figure 1 shows a graph of a hypothetical standby load profile. Although the genset is loaded to 90% of its standby rating for a portion of the time, the average load factor over time is only 70% due to the natural variability of the building load.

Figure 1: In this graph of a hypothetical standby load profile, the 24-hour average load factor is derived from the graph, where P is power in kW and t is time. Courtesy: MTU Onsite Energy

In practice, it would be unlikely that a standby power system would be initially sized so small as to require operating at 100% of capacity at any time during an outage. However, electrical loads are often added, and growing power needs may begin to tax the capacity of a standby power system. It should be noted that any time that the genset is offline does not count toward the 24-hour average load factor.

High mission-critical load factors

For most facilities with properly designed emergency standby power systems, the possibility of exceeding a power system’s 24-hour average load factor limitation is remote. This is because most commercial facilities have variable load profiles that reduce the likelihood a power system’s 24-hour average load factor limitation will be exceeded, even during an extended outage. Many facilities also have noncritical loads that can be taken offline during extended outages to reduce the average load factor on the standby system, if necessary.

However, many mission-critical facilities have large, less-varying loads that can severely stress standby power systems during an extended power outage unless steps are taken during system design to accommodate the potential for a higher average load factor. Two examples of mission-critical facilities with high load factors are data centers and semiconductor manufacturing. In data centers, the computer server and HVAC equipment create high electrical loads that can vary little over time. Similarly, very high load factors are found in semiconductor foundries, where electric furnaces cannot be shut down without destroying large amounts of product.

Figure 2: This graph shows the load profile of gensets capable of an 85% load factor. Although the gensets are not loaded to 100% of their standby rating at any time, the average load factor during the outage is near 85%. Courtesy: MTU Onsite Energy

Because of these large, steady electrical loads, the load profile in a mission-critical application is likely to have less variability, in turn putting a more constant demand on the standby power system. Less load variability results in a higher average load factor that will require specifying a system with larger or more gensets capable of a 70% load factor or specifying gensets capable of higher than a 70% load factor.

Figure 2 shows that while the gensets are not loaded to 100% of their standby rating at any time, the average load factor during the outage is near 85%. In this case, the customer has taken advantage of gensets capable of an 85% load factor that can deliver more than 20% additional kW than gensets rated to only a 70% average load factor.

Defining genset standards

The International Organization for Standardization (ISO) has established standards that apply to all gensets. ISO defines how to measure and rate many quality and performance parameters. All major genset manufacturers use this standard to communicate their genset ratings to their customers. In particular, ISO 8528-1 describes how to establish genset ratings; measure performance; and evaluate engines, alternators, controls, and switchgear.

ISO 8528-1 sets a maximum 24-hour average load factor capability of 70% for both standby- and prime-rated gensets, unless a higher average is agreed to by the engine manufacturer. This means that a 3,000 kW genset meeting this standard must be able to provide an average of 2,100 kW per hour over a 24-hour period. In emergency standby applications, this means that the average load factor that can be sustained by most gensets over an extended outage of 24 hours or more must not exceed 70% of the nameplate standby rating, a factor that affects genset sizing.

One genset manufacturer allows an 85% average load factor on emergency standby-rated genset models above 200 kW. For example, the genset manufacturer allows its 3,250 kW genset to deliver a 24-hour average of 2,762.5 kW in this case. For certain applications involving multiple gensets, this higher average load factor capability may reduce the number of gensets needed to supply the load.

ISO 8528-1 defines four genset power output rating categories. They are:

1. Emergency standby power (ESP) rating: The ESP rating is the maximum amount of power that a genset is capable of delivering. It is normally used to supply facility power to a variable load in the event of a utility outage. No overload capacity is available for this rating. ISO 8528-1 limits the 24-hour average output to 70% of the nameplate ESP rating unless the manufacturer allows a higher average load factor.

2. Prime-rated power: A prime-rated genset is available for an unlimited number of hours per year in a variable-load application as long as the average load factor does not exceed 70% of the nameplate rating, unless the manufacturer allows a higher average load factor. This rating allows an overload capacity of 10%, but that additional capacity should not be used for more than one hour in every 12. The prime power rating for a given genset is typically 10% lower than the standby rating.

3. Limited-time running power: This rating is a subset of prime power and allows prime power to be available for a limited number of hours in a nonvariable load application. It is intended for use in situations where power outages are contracted (utility power curtailment or peak shaving). Gensets can be paralleled with the public utility up to 500 hours per year at 100% of the prime rating. However, be aware that the life of the generator drive engine may be reduced by this constant high load operation. Any application that operates at the prime power rating for more than 500 hours per year should use a larger genset at its continuous rating.

4. Continuous output power rating: The continuous power rating is used for applications where there is no utility power and the genset is relied upon for all power needs. Gensets with this rating are capable of supplying power at a constant 100% of rated load for an unlimited number of hours per year. No overload capability is available for this rating. The continuous power rating for a given genset is typically 25% to 30% lower than the standby rating.

Effects of load factor on power system design

Specifying standby gensets with a higher-than-average load factor capability can sometimes be a benefit in mission-critical applications. System designers may be able to reduce the size or number of gensets by using units approved for 85% average load factor, as opposed to the 70% average load factor. For example, to design a standby power system to supply an average load of 11,000 kW at a 70% average load factor would require eight 2,000 kW gensets. At a 70% average load factor rating, each genset would be able to deliver up to a 1,400 kW average, for a total capacity of 11,200 kW over an extended outage of 24 hours or more.

8 x 2,000 kW x 0.70 = 11,200 kW

Using gensets with an 85% average load factor capability would require only seven 2,000 kW units. Each genset would be able to deliver up to a 1,700 kW average, for a total average of 11,900 kW over an extended outage of 24 hours or more. That amounts to an extra 2,100 kW of effective generating capacity for extended outages and a reduction by one in the number of gensets needed.

7 x 2,000 kW x 0.85 = 11,900 kW


The load factor of any application affects the design and sizing of the standby power system. However, for mission-critical applications, particular attention must be paid to load factors because of the minimal ability of some facilities to reduce their electrical loads during extended outages.

While all major manufacturers of gensets use ISO-8528-1 as their standard, which sets the average 24-hour load factor at 70%, system designers can choose equipment that offers a higher average 24-hour load factor, which may result in a system with smaller and/or fewer gensets. Specifiers of standby power systems for mission-critical applications should understand average load factor and its implications for business continuity in the face of natural or manmade disasters.

Kraemer is application engineering manager at MTU Onsite Energy, Mankato, Minn. He has a degree in automotive engineering from Minnesota State University and leads MTU Onsite Energy’s new product development and custom application projects, while providing technical application support for all genset models.