A S H RA E
JOURNAL
The following article was published in ASHRAE Journal, December 1997. © Copyright 1997 American Society of Heating, Refrigerating and Air-Conditioning Engineers,
Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.
Less Than One Watt per Square Foot
Space Cooling Demands
From Office Plug Loads
By Paul Komor, Ph.D.
1. Using actual information based on
expected building use, or
2. Using manufacturer’s published
data or information from technical societies, or
3. “Other data based on the designer’s
experience of expected loads and occupancy patterns.”2
This first option often isn’t possible for
The third option, based on experience
and accepted practice, is the most common. Informal discussions with engindersizing space cooling systems
neers and HVAC system designers
for office buildings can result in
reveal that typical assumptions are in the
uncomfortable and angry tenants on peak
2 to 5 watts per square foot (21.5 to 53.8
cooling days. However, oversizing
W/m2) range—a bit lower than ASHwastes money because more capacity is
RAE’s upper boundary. Some are a bit
installed than is needed, and oversized
higher. In the current $1.2 billion renosystems have a lower
vation of the Pentagon
energy efficiency which
(a 6.5-million ft2
makes operating costs
[604,000 m2] governhigher than necessary.
ment office building
Oversizing
can
outside Washington,
adversely affect comD.C.), initial specififort as well, because
cations call for an
oversized systems may
internal
equipment
provide poor humidity
load assumption of
control and large tem5.5 watts per square
perature variations.
foot (59.2 W/m2) for
Correct system sizopen office space.4
ing requires estimating
Fortunately, enough
building heat loads
measurements of plug
accurately. Many facloads are now available
tors contributing to
to replace assumptions
building heat loads
with measured data.
must be considered
These data suggest that
when sizing cooling
nameplate ratings do
Table 1: Energy use of office equipment in a typical 50-person office.
systems. Space cool- Power consumption numbers are based on measured data for 1995 not indicate actual
ing systems must be average stock equipment (not nameplate ratings).
power use and that plug
sized to remove heat
loads are typically
and moisture from
between 0.4 and 1.1
both external loads (such as solar gain office equipment, because it is usually not watts per square foot (4.3 and 11.8 W/
and outside air) and internal loads— known what types of equipment will be m2)—far below the 2 to 5 watts per square
lighting, occupants, and plug loads.
used in the space. Even if the building foot (21.5 to 53.8 W/m2) commonly
The plug loads category includes any will be owner-occupied, final decisions assumed.
electrical equipment that is plugged into about office equipment are almost always
Actual Heat Output Not Indicated
outlets. These loads typically account made after construction is completed.
for about 15 to 20% of total cooling
The second option is of limited use by Nameplate Ratings
load.1 For office buildings, the plug for office equipment, because until now
Several studies have examined the
loads that use the most energy are com- there have not been sufficient data avail- relationship between nameplate ratings
puters and related equipment such as able for technical societies to provide
printers, copiers, and monitors (See guidance. The 1993 ASHRAE HandTable 1). The proliferation of these book—Fundamentals notes that “in
About the Author
devices in offices has become an offices having computer display termiincreasing concern in recent years.
Paul Komor, Ph.D., is research managnals at most desks, heat gains range up to
Little guidance is available for esti- 15 Btu/h•ft2 (4.4 watts per square foot
er at E Source, Inc., an energy research
mating the magnitude of these plug [47.3 W/m2]).”3 This provides a valuand information firm; and is also a facloads. California’s Title 24 standard, for able upper boundary, but does not proulty member at the University of Coloexample, gives three options for calcu- vide the detailed guidance necessary to
rado in Boulder.
make reasonable assumptions.
lating miscellaneous equipment loads:
U
December 1997
ASHRAE Journal
41
and actual power use.5 These comparisons, summarized in Figure 1, show that
actual measured power use is typically
20 to 50% of the nameplate rating.
Nameplate ratings are intended for use
in sizing electrical wiring. These ratings
are not intended for use in calculating
actual non-instantaneous power use or
heat output. There may be times when
nameplate power is drawn for very short
periods (for example, when starting
equipment). However, these short-term
power draws do not produce appreciable
amounts of heat, and therefore should not
influence cooling system sizing decisions.
Actual Power Demand:
The Evidence
In the last few years, a number of
building researchers and engineers have
taken real-time measurements of plug
load power use. This is difficult because
the wiring to outlets is often on the same
circuit as wiring to overhead lighting.
This makes it challenging to separate out
the outlet power. Also, office equipment
contains power supplies that can produce third harmonics. These can be difficult to measure but should be included
because they contribute to heat output.
Despite these challenges, enough
high-quality measured data on plug loads
exist to substitute real data for informal
guidelines. The data agree with each
other—but differ from commonly-used
assumptions and guidelines. As shown in
Figure 2 and Table 2, measured data on
actual plug loads range from 0.4 to 1.1
watts per square foot (4.3 to 11.8 W/
m2).6 These data are drawn from 44
buildings, covering a total of 1.3 million
ft2 (121,000 m2). The buildings were
selected for measurement because they
represent “typical” office buildings.
Although the results may seem low,
consider the type of equipment found in
a typical office and the amount of
power it actually consumes. For example, assume that an office with a staff of
50 people has a computer with color
monitor for each employee, and that
there are five networked printers, three
copy machines, and three fax machines.
In this office there are also two refrigerators, two microwaves, and 10 freestanding incandescent lamps in the
office. Total power demand for these
devices, as shown in Table 1, adds up to
10.2 kW. (No allowance is made for
diversity because all devices are
assumed to be always turned on.) Also,
assume 200 ft2 (19 m2) of conditioned
space per person (this includes meeting
rooms, hallways, bathrooms, and other
common space). This scenario yields
10,000 ft2 (930 m2), or 1.02 watts per
square foot (11.0 W/m2). This is consistent with the measured data shown in
Figure 2.
Benefits of “Rightsizing”
Assuming one watt per square foot
for plug loads, as the data suggest is
appropriate—rather than the more typical three watts per square foot—can
make a significant difference in cooling
Table 2: Office equipment plug loads data. Data were compiled from both published and unpublished sources. Unless noted otherwise, all measured watts per
square foot data include all plug loads.
42
ASHRAE Journal
Fig. 1: Nameplate vs. actual power
demand. Each dot indicates a separate study. For example, actual power demand for computers was found
to be 14 to 33% of nameplate power
demand. Study results differ largely
because different brands with different internal equipment were tested.
system sizing and costs. For example,
assuming three watts per square foot
(32.3 W/m2) for plug loads in a 100,000
ft2 (9,300 m2) office building yields a
300 kW office equipment load. Removing the heat from this load would require
85 tons (300 kW) of effective cooling
capacity. Assuming one watt per square
foot, however, corresponds to about 28
tons (100 kW) of effective cooling
capacity. The difference (57 tons [200
kW]) translates into a significant cost
savings.
Chiller capacity alone costs about
$400 per ton ($114/kW),7 so an accurate
estimation of plug loads would save
about $23,000 in up-front chiller capacity
costs. (Cooling system inefficiencies
require more than one ton of chiller to
remove one ton of load, so actual savings
would be a bit higher.) For new construction, additional savings of up to $3,000
per ton ($850/kW) may be possible by
equivalent downsizing of ducts, fans, and
other cooling-related equipment.8
The operating cost savings of “rightsizing” are significant for packaged
December 1997
OFFICE BUILDING
rooftop units and chillers. Packaged
units are often inefficient during start up,
and reach maximum efficiency after
about 10 minutes.9 Therefore, an oversized packaged unit that cycles frequently will spend more time operating
in its inefficient start-up period. For
packaged rooftop units and chillers,
there is an efficiency penalty for operating at low load.10 Rightsizing chillers
forces the typical operating conditions
into the efficient medium to high load
range, which can result in operating cost
savings.
Rightsizing cooling plants has other
advantages as well:
• A properly-sized cooling plant cycles less frequently, keeping indoor temperature constant and providing better
humidity control.
• All else being equal, maintenance of
smaller units is simpler and cheaper
(maintenance contracts are typically
charged by the ton).
• In retrofits, smaller units free up
electrical capacity that can be used for
other needs.
The data shows that plug loads are
considerably lower than commonly
thought. It is important, however, to recognize that these are average officewide loads. Certain locations such as
kitchens and copy rooms can have much
higher plug loads. Although total cooling capacity should be sized to handle
average loads, air handling and distribu-
tion systems need to be flexible enough
to handle higher loads if necessary. The
use of diversity factors in load calculation software is a useful method to limit
oversizing of cooling plants.
Plug Loads Decreasing
New HVAC systems in commercial
buildings should be sized not only to
accommodate the building’s current
loads, but to handle expected future
loads as well. Future plug loads will
depend on equipment density (the number of computers, printers, and other
devices per square foot), hours of use per
year for each piece of equipment, and
energy use for each piece of equipment.
Although these variables are uncertain, some general trends are likely. In
the United States, the Energy Star program (a U.S. government-sponsored
effort to encourage office equipment
manufacturers to increase energy efficiency) probably will cause significant
reductions in per-device energy use for
computers, monitors, printers, copiers,
and fax machines sold in the United
States. Similar programs are in place or
being discussed in Australia, Japan,
New Zealand, Sweden, and Switzerland.
Forecasts of equipment density for
the United States show computer and
monitor density continuing to grow, but
at a declining rate—reaching about one
computer and monitor per person by
about 2000.11 (Of course, some offices
already may have more than one computer per person, but the average U.S.
office currently is at about 0.7 per person).12
Hours of use per year and person density (square feet per person) are unlikely
to change significantly.
The result of these factors, according
to a recent study in the United States,13
will be a decrease in office equipment
energy use intensity (kWh per square
foot per year) from now until about 2002
(see Figure 3). Forecasting is an uncertain business, but there is little evidence
that office equipment power densities
(watts per square foot) will increase significantly in the next decade. Rather,
they will likely decrease, partly because
of technical advances promoted by the
Energy Star and other similar programs.
Conclusions
Plug loads require significant cooling
capacity. Moderate oversizing for plug
loads (in case tenants are unusually
equipment-intensive) is reasonable. A
useful guide for such oversizing is considering how outdoor design conditions
are determined: a typical assumption is
to use 2.5% conditions, meaning that the
cooling system capacity is designed to
provide comfort in all but the hottest
and/or most humid 2.5 hours out of
100.14 By analogy, the data shown in
Figure 2 have a mean (µ) of 0.83 watts
per square foot (8.9 W/m2) and a sample
Fig. 2: Office equipment plug loads. This figure shows measured office equipment power use (in watts per square foot)
for 44 buildings comprising 1.3 million ft2 (121 000 m2). The simple average is 0.83 watts per square foot (8.9 W/m2).
December 1997
ASHRAE Journal
43
standard deviation (σ) of 0.21 watts per square foot (2.3 W/
m2). Assuming the data are normally distributed, therefore,
suggests that only 2.5% of the data will have a value greater
than µ+2σ, or 1.25 watts per square foot (13.4 W/m2).
When building new facilities or replacing existing cooling
systems, energy users should carefully examine assumptions
for office equipment energy use. If plug loads greater than 1.25
watts per square foot (13.4 W/m2) are specified, there should
be a compelling reason for doing so—such as clear evidence
that actual loads in the space will be much higher than typical
loads. In such cases, these loads should not be overestimated.
Even in very equipment-intensive settings such as commodity
trading floors, plug loads have been measured at three watts
McGaffin, “Measuring Computer Equipment Loads in Office Buildings,” ASHRAE Journal (August 1994).
6. Dubin-Bloome Associates, P.C., “Conceptual Design Report for
Pacific Gas and Electric Customer Technology Test (ACT 2 ) For
Energy Efficiency,” prepared for PG&E (January 1991); J. Farley,
personal communication (March 1996), Center for Electric End-use
Data (CEED), EPS Solutions, 1 Richmond Square, Suite 122C, Providence, RI 02906; Larry Lister (March 1996); E. Martin, “Energy
Efficiency of Electronic Office Equipment: Case Study for a Building
Retrofit,” Proceedings of the 1992 ACEEE Summer Study on Energy
Efficiency in Buildings, ACEEE, Washington, DC, v. 1, pp. 167 Load
Equipment Energy Use in the Richard Blanshard Building, 1992 and
1994,” prepared for British Columbia Buildings Corporation (May
1995); and Wilkins and McGaffin (1994).
7. Author’s estimate based in part on Means Mechanical Cost
Data, 19th Edition (1995), p. 281.
8. Martin Kiley and William Moselle, eds., National Construction
Estimator, 40th Edition (Carlsbad, CA: Craftsman Book Company,
1992), p. 476.
9. J. Proctor, Z. Katsnelson, and B. Wilson, “Bigger Is Not Better:
Sizing Air Conditioners Properly,” Refrigeration Services and Contracting, v. 64, no. 4 (April 1996), p. 24.
Fig. 3: Forecast of office equipment energy intensity. Energy use intensity is expected to decrease until about 2002,
and to rise slowly thereafter.
per square foot (32.3 W/m2), rather than the 8 to 10 watts per
square foot (86.1 to 107.6 W/m2) specified.15
10. S. Silver, P. Fine, and F. Rose, “Performance Monitoring of DX
Rooftop Cooling Equipment,” Energy Engineering Vol. 87 No. 5,
1990, pp. 32-41; L. Fryer, “Electric Chiller Buyer’s Guide,” E Source
Tech Update TU-95-1, February 1995, p. 17.
11. J. Koomey, M. Cramer, M. Piette, and J. Eto, “Efficiency
Improvements in U.S. Office Equipment: Expected Policy Impacts
and Uncertainties,” LBNL, Berkeley, CA (December 1995), p. 10.
References
12. Koomey et al. 1995, p. 10.
1. See, for example, M. Shepard et al., Commercial Space Cooling
And Air Handling Technology Atlas, June 1995, E Source, Boulder,
CO, p. 3.1.
13. Koomey et al. 1995.
2. California Code of Regulations, Title 24, Part 6, Subchapter 5,
Section 144(a)9. This document is available at www.energy.ca.gov/
reports/title24.
3. 1993 ASHRAE Handbook—Fundamentals, p. 26.11.
4. Larry Lister, personal communication (May 1996), Construction Engineering Research Laboratories (CERL), U.S. Army Corps of
Engineers, P.O. Box 9005, Champaign, IL 61821-9005.
5. G. Newsham and D. Tiller, “The Energy Consumption of Desktop Computers: Measurement and Savings Potential,” IEEE Transactions on Industry Applications, v. 30, no. 4 (July/August 1994), pp.
1065-1072; M. Piette, J. Eto, and J. Harris “Office Equipment Energy
Use and Trends,” Lawrence Berkeley Laboratory, LBL-31308 (September 1991); R. Szydlowski and W. Chvála, “Energy Consumption
of Personal Computer Workstations,” Battelle/Pacific Northwest
Laboratory, PNL-9061 (February 1994; and C. Wilkins and N.
44
ASHRAE Journal
14. For more information, see 1993 ASHRAE Handbook—Fundamentals, chapter 24.
15. Scott Frank, personal communication (June 1996), Jaros Baum
and Bolles Consulting Engineers, 345 Park Avenue, New York, NY
10154-0002.
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