TECHNICAL ARTICLE

GENERATOR SET LOAD FACTOR IMPLICATIONS

FOR SPECIFYING ONSITE GENERATORS

One of the important steps in sizing generator sets for any application
is to determine the application’s average load factor. Understanding this
parameter is essential not only for proper power system sizing but also for
operability and reliability.

When specifying an engine generator set there There are three key attributes which ISO-8528
are a number of parameters to be identified, and uses to define each power rating category. They
among them is the generator set rating. When are:
choosing a rating, it is important to understand
that implicit with it is a value called Permissible // Load Profile — This defines whether the
Average Power, or what is commonly referred to as generator serves a constant electrical load,
average load factor. This article will explore how or a variable electrical load.
average load factor and generator set ratings are
// Annual Run Time — This defines the number
related, and how these parameters come into play
when choosing a generator set to meet a particular of hours the generator is expected to run
application. each year.

// Permissible Average Power — This is the average

DEFINING STANDARDS FOR
load over any 24-hour period of operation.
GENERATOR SETS
So here we see that when a power rating is
Most major manufacturers of generator sets
chosen, along with it comes a predefined
use published standards to guide the design
permissible average power. We will focus
and manufacture of their products. Among
on this average load power. For a complete
these standards are those established by the
discussion about power ratings please refer
International Organization for Standardization
to Understanding Generator Set Ratings.
(ISO). Within ISO standard 8528 (particularly
part one) we find the definition of four different
power rating categories; Continuous, Prime,
Emergency Standby, and Limited Time Running.
Although ISO-8528 defines all four of these
ratings, for the purposes of this article, we will
only be discussing the first three as they are the
most common.

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02/ // / UNDERSTANDING POWER SYSTEM LOAD FACTOR

ISO-8528 defines categories of generator set power output ratings FIGURE

FIGURE 1. AVERAGE LOAD1. AVERAGE
FACTOR LOAD FACTOR
// Emergency Standby (ESP) Rating — The ESP rating is the maximum
amount of power that a generator set is capable of delivering, and it is
normally used to supply facility power to a variable load in the event 90
of a utility outage. ISO-8528-1 limits the 24-hour average output to 70 80
percent of the nameplate ESP rating. Figure 3 shows a typical load 70 70 % of rated power (P)
profile for an ESP-rated generator set.
60
// Prime-RatedPower (PRP) — A prime-rated generator set is available for 50
an unlimited number of hours per year in a variable-load application,
as long as the average load factor does not exceed 70 percent of the
nameplate rating. The prime power rating for a given generator set is
typically 10 percent lower than the standby rating. Figure 4 shows a
typical load profile for a PRP-rated generator set.

// ContinuousPower Rating (COP) — The continuous power rating is used

0 4 8 12 16 20 24 hours
for applications where there is no utility power and the generator set
is relied upon for all power needs. Generator sets with this rating are t1 t2 t3 t4 t5 t6 time (t)
capable of supplying power at a constant 100 percent of rated load for
an unlimited number of hours per year. The continuous power rating (P1 x t1)+(P2 x t2)+(P3 x t3)+(P4 x t4)+(P5 x t5)+(P6 x t6)
ALF =
t1 + t2 + t3 + t4 + t5 + t6
for a given generator set is typically 25-30 percent lower than the
(90 x 4)+(70 x 4)+(80 x 4)+(50 x 4)+(60 x 4)+(70 x 4)
standby rating. Figure 5 shows a typical load profile for a COP-rated ALF =
4+4+4+4+4+4
generator set. 360 + 280 + 320 + 200 + 240 + 280 1680
ALF = = = 70%
24 24
With these definitions, if a manufacturer states that a particular model
meets the requirements of ISO-8528, a customer already knows or can
easily reference the performance to be expected from that model. However,
In Figure 1, the 24-hour average load factor is derived from the formula
although ISO-8528 clearly defines each of the power ratings and their
shown under the graph, where P is power in kW and t is time. You can
associated average load factor, it should be noted that the ISO standard
see that although the generator set is loaded to 90 percent of its standby
only serves as a starting point for an agreement between a manufacturer
rating for a portion of the time, the average load factor over the 24 hour
and customer. What this means is that manufacturers can (and do) deviate
time span is only 70 percent. This is due to the natural variability of the
from the standard, but if a manufacturer deviates from the standard, it
building load found in many applications where standby generators are
should be agreed on by both the manufacturer and customer.
installed. Overnight, loads are light because lights are turned off, and
much of the equipment is not running. Then, during the morning and
When it comes to average load factor, there are two common deviations
throughout the day the loads increase as lights are turned on, equipment
that some manufacturers make. While the ISO standard allows for a 70
is put into use, and things like air conditioners are running more often.
percent average load factor in ESP and PRP ratings, some manufacturers
For many application, this variation of load is enough to ensure that
allow for higher values, even as high as 85 percent. The second is an
average load factor limits are not exceeded, but it should be looked at
allowance for a 10 percent overload capability for generator sets with a
for each new installation. Consideration should also be given to future
prime power rating. A little later in our discussion we will come back to
expansion, and how the load factor may change over time.
how all of these numbers can affect generator sizing, but first we should
clearly define average load factor, and how it can be calculated.
Note also that the calculation for average load factor is done using a
minimum of 30 percent load, so even when loaded under this value,
AVERAGE LOAD FACTOR
30 percent is used in the calculation. And finally, only operating hours are
The average load factor of a power system is determined by evaluating the used in the calculation, any time that the generator set is offline does not
amount of load and the corresponding amount of time the generator set is count towards the 24-hour average load factor.
operating at that load. In the case of a power system supplying a constant
load this calculation can be done simply by dividing the power supplied by
the generator set rating. In most cases however, the loads are variable, so
the calculation must be broken into time segments as shown in Figure 1.

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03/ // / UNDERSTANDING POWER SYSTEM LOAD FACTOR

FIGURE 2. MISSION-CRITICAL LOAD PROFILE

FIGURE 2. MISSION-CRITICAL LOAD PROFILE
FIGURE 3. TYPICAL ESP LOAD PROFILE
FIGURE 3. TYPICAL ESP LOAD PROFILE

90 90 90 90
85 85 % of rated power (P)
80
70

50 50 % of rated power (P)

30 30

0 4 8 12 16 20 24 hours
0 4 8 12 16 20 24 hours
t1 t2 t3 t4 t5 t6 time (t)
t1 t2 t3 t4 t5 t6 time (t)
360 + 200 + 320 + 120 + 120 + 200
ALF =
24
Average Load Factor = 85%
1320
ALF = = 55.00%
24

HIGH MISSION-CRITICAL LOAD FACTORS EFFECTS OF LOAD FACTOR ON POWER SYSTEM DESIGN
For most facilities with properly designed emergency standby power Specifying standby generator sets with a higher-than-average load fac-
systems, the possibility of exceeding a power system’s 24-hour average tor capability can sometimes be a benefit in mission-critical applications.
load factor limitation is remote. This is because most commercial facilities System designers may be able to reduce the size or number of genera-
have variable load profiles that reduce the likelihood a power system’s tor sets by using units approved for 85 percent average load factor, as
24-hour average load factor limitation will be exceeded, even during an opposed to the 70 percent average load factor. For example, to design a
extended outage. Many facilities also have noncritical loads that can be standby power system to supply an average load of 11,000 kW at a 70
taken offline during extended outages to reduce the average load factor percent average load factor would require eight 2,000 kW generator sets.
on the standby system, if necessary. At a 70 percent average load factor rating, each generator set would be
able to deliver up to a 1,400 kW average, for a total capacity of 11,200 kW.
However, many mission-critical facilities have large, less varying loads that
if not properly considered can exceed the published load factor of standby 8 x 2,000 kW x .70 = 11,200 kW
power systems. Two examples of mission-critical facilities with high load
factors are data centers and semiconductor manufacturing. In data centers, Using generator sets with an 85 percent average load factor capability
the computer servers and HVAC equipment create high electrical loads would require only seven 2,000 kW units. Each generator set would be
that can vary little over time. Similarly, very high load factors are found able to deliver up to a 1,700 kW average, for a total average of 11,900 kW.
in semiconductor foundries, where electric furnaces cannot be shut down That amounts to an extra 700 kW of effective generating capacity and a
without destroying large amounts of product. reduction by one in the number of generator sets needed.

As a result of these large, steady electrical loads, the load profile in a 7 x 2,000 kW x .85 = 11,900 kW
mission-critical application is likely to have less variability, in turn
putting a more constant demand on the standby power system. Less load CONCLUSION
variability results in a higher average load factor that will require either
specifying a system with larger or multiple generator sets or specifying The load factor of any application affects the design and sizing of the
generator sets capable of a higher average load factor. standby power system, but for mission-critical applications, particular
attention must be paid to load factors because of theses facilities’ near
In Figure 2, you can see that while the generator sets are not loaded to constant load and limited ability to reduce their electrical loads. While
100 percent of their standby rating at any time, the average load factor all major manufacturers of generator sets utilize ISO-8528-1 (which sets
during the outage is near 85 percent. In this case, the customer has taken the average 24-hour load factor at 70 percent) as their standard, system
advantage of generator sets capable of an 85 percent load factor which can designers can choose equipment that offers a higher average 24-hour
deliver over 20 percent more power on average than generator sets rated load factor, which may, in turn, result in a system with smaller and/or
to only 70 percent average load factor. fewer generator sets. In any case, those specifying standby power systems
need to understand average load factor and its implications for business
continuity in the face of natural or man-made disasters.

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04/ // / UNDERSTANDING POWER SYSTEM LOAD FACTOR

FIGURE 4. TYPICAL
FIGURE 4. TYPICAL PRP LOAD PROFILE
PRP LOAD PROFILE FIGURE 5. TYPICAL COP LOAD PROFILE
FIGURE 6. TYPICAL COP LOAD PROFILE
110
100 100 100 % of rated power (P)
100

80

60 60 % of rated power (P)

50 50

t1 t2 t3 time (t)
0 4 5 8 12 16 20 24 hours
Average Load Factor = 100% of COP rating
t1 t2 t3 t4 t5 t6 t7 time (t)

1630
ALF = = 67.92%
24

ISO-8528 sets the 24-hour average load factor for a

generator set at 70 percent of the nameplate rating. While
some generator set manufacturers allow a higher 24-hour
average load factor under certain circumstances, MTU
Onsite Energy allows an 85 percent 24-hour load factor on
MTU Onsite Energy all its standby generator sets (from 230 kW to 3,250 kW).
Part of the Rolls-Royce Group

www.mtuonsiteenergy.com

MTU Onsite Energy is part of the Rolls-Royce Group. It provides diesel and
gas-based power system solutions: from mission-critical to standby power
to continuous power, heating and cooling. MTU Onsite Energy power systems
are based on diesel engines with up to 3,250 kilowatts power output (kWe)
and gas engines up to 2,530 kWe.

©2017/ // / MTU Onsite Energy 10 053 (77 3E) 16/01