Shedding Light on Home Lighting Use

The most extensive lighting monitoring study ever indicates that lights in living rooms, kitchens, and outdoors get the most use.

Place enough lighting loggers in enough homes for a long enough time and you should get a good idea of how long people leave their lights on. If you write down what kind of lamps are in the fixtures, you can also figure out how much energy residents use on lighting. Break down the results by room, and you can even start to tell which rooms have the best opportunities for lighting retrofits.

I recently led a project at Tacoma Public Utilities to accomplish these goals. We learned a fair bit about how much residential energy is used for lighting and what rooms are most often lit up. While the sample is not big enough to extrapolate to all homes, the results generally agree with other lighting use studies. The growing body of information will be useful for retrofitters and utilities hoping to maximize savings from lighting retrofits.

Utility conservation program managers in the Pacific Northwest were eager to add compact fluorescent lamps (CFLs) to the energy-saving products they could offer their residential customers. They realized, though, that they didn’t know much about how their customers used lights at home. Previous studies of residential lighting energy use relied on manufacturers and customer self-reports to estimate use. One measured-use study had been done, but it only monitored a couple dozen homes for three months.

Tacoma Public Utilities, in cooperation with Portland General Electric, Eugene Water and Electric Board, City of Port Angeles, Pacific County Public Utility District Number 2, Snohomish County Public Utility District, and Peninsula Power and Light, designed a study to learn more about single-family customers. Each utility chose customers to represent a cross-section of their service area–a mixture of urban, suburban, and rural areas with a variety of demographic characteristics.

The study sought to answer the following questions:

  • How much lighting energy does each residence consume?
  • What percentage of fixtures have lamps in use three hours or more per day? In what rooms or areas are these high-use lamps concentrated?
  • What will be the annual residential energy savings from replacing 50W-150W incandescent lamps with CFLs?
  • How is lighting use affected by heated square footage, number of occupants, and hours of occupancy?

Monitoring the Lights

Utility conservation program managers thought it sounded easy enough. We estimated that there would be about two dozen fixtures per home, and they considered it feasible to monitor 50 houses for six months at a time. Over two years, that would be about 200 homes. We bought 1,235 Pacific Science and Technology run-time loggers for the study. (Run-time loggers electronically measure how long a piece of equipment has been on. The ones used in this study have light sensors, so they log how much time a light is on.)

In the end, we collected data from 82% of the lights in 161 homes over about two years, recording how much time each light was on. Some of the homes were monitored for 12 months, though most were only monitored for 4 to 6 months. Due to the size of the study, the 6-month periods did not all start or finish at the same time (see Figure 1). However, some periods were during generally lighter months (early February to late August), while other periods were generally darker (mid-July to early February, or early September to the end of February). This distinction proved to be important in the results.

Figure 1. The relationship between monitoring periods and dark and light portions of the year.

Energy auditors installed loggers and inventoried existing lamps, fixtures, and switches. They also gathered information regarding the number of residents and the size of the heated area. Loggers were installed on 82% of the houses’ 4,813 fixtures, both indoors and out. When there were not enough loggers to measure all the fixtures in a house, auditors asked the residents which fixtures they almost never used; 858 such fixtures were left unlogged.

Auditors installed the loggers as inconspicuously as possible and told the customers to keep using their lamps as they always had. Aluminum-clad fiber-optic extensions were used to keep the loggers from being affected by light from the sun or other lamps. These extensions also let auditors mount loggers a couple of feet from their light sources to prevent chandeliers from tipping off balance, to preserve fixtures’ aesthetics, or to prevent loggers from melting. Some loggers were fastened to fixtures with nylon wire ties, hook-and-loop fasteners, insulated wire, or magnets. Others hung from tiny hooks screwed into the ceiling.

The installers often set the loggers’ sensitivity adjusters near maximum when they used fiber-optic extensions. Not much light reached the sensor, especially when the extension was very bent. Another kind of problem occurred when measuring outdoor reflective flood lamps. Sometimes the loggers counted extraneous events, such as the reflections of car headlights, as if the lights had turned on, but these events were probably only momentary, and probably didn’t affect the long-term run time by much.

Every month, energy auditors read the loggers and checked to make sure that they were operating properly. After entering the data into their computers, they sent it to an electronic bulletin board, where I downloaded it. Once they got the hang of it, this was a convenient and inexpensive way to transfer data.

Customers said the loggers didn’t get in the way of their activities at all. In one case, a logger was even painted purple with the rest of the bathroom. In another home, the teenagers’ bedroom was relit with black lights for their posters. The energy auditors simply noted the change and adjusted the loggers.

Reading the Results

After all that time working with the loggers and data, we were able to draw the following conclusions:

  • Average residential lighting energy use in this area is about 1,800 kWh per year per household.
  • 50% more energy is used for residential lighting in the darker months than in the lighter months.
  • The locations with the highest lighting energy use are living rooms, kitchens, porches, and outdoors. This matches results of studies from the Lighting Research Center (see “Enlightening Results from Other Research“) and the Florida Solar Energy Center (see “Florida House Aglow with Lighting Retrofit,” p. 21).
  • Incandescent lamps in the 50W to 150W range that were used three or more hours per day could be replaced by $15 CFLs. This would save an average of $5.60 per year at 4¢/kWh (Tacoma’s rates) with a simple payback of 2.7 years.

We were surprised to find that lighting energy use did not correlate with heated floor area, number of occupants, or hours of occupancy. Apparently, behavior and other occupant factors had more of an effect than the size of a home and the number of people present. When it came to lighting, some customers had strong conservation habits and others did not. The LRC survey also found no correlation between hours of use and type of home or demographic characteristics. 

How does measured lighting energy use compare to total household energy use? During the periods the loggers were installed, the households used an average of 14,900 kWh overall. Lighting energy use for these same periods averaged 1,370 kWh, or approximately 9% of the total. This data was not weighted according to whether homes had electric or non-electric space heating or to the time of year, and lighting usage varied greatly between homes. Therefore, the percent of total usage should be considered a rough estimate.

There were significant differences in usage between the lighter and darker periods of the year. In the darker periods, the number of households using fewer than 1,000 kWh for lighting went down dramatically. The number of homes using 1,000-1,999 kWh per half year also dropped significantly. Daily lighting energy use leapt from 4 kWh to 6 kWh between the lighter and darker periods.

Table 1. Percentage of Fixtures On at
Least Three Hours per Day, by Location
Location Darker Lighter
Period Period
Living rooms 44% 27%
Kitchens 52% 33%
Porches 48% 34%
Bathrooms 14% 19%
Bedrooms 14% 8%
Master bedrooms 16% 8%
Yard/driveway 30% 17%
Household 27% 19%

Table 1 shows fixture use by room. About 44% of the fixtures in living rooms and 52% of the fixtures in kitchens were on three hours or more per day during the darker periods. Only 14% of the fixtures in bathrooms were on three hours or more per day. The low hours of use in bathrooms surprised utility program managers, who had been encouraging builders to install fluorescent fixtures in bathrooms. The average on time for bathrooms turned out to be only 1.7 hours per day.

Consequences for Auditors

There is a lot of variation in the way people use lights. More than half the living room fixtures were used fewer than three hours per day. In fact, 40% of the logged fixtures were on less than one hour per day. Meanwhile, other lights were left on a lot. According to Judith Jennings and other researchers at Lawrence Berkeley National Laboratory who interpreted this study’s data, 30% of the incandescent fixtures were responsible for over 80% of the incandescent lighting energy use.

Auditors looking for noticeable short-term savings can be guided by the usage patterns in this study, but should always ask the customers about their own usage before replacing lamps. A room with more light fixtures is not necessarily the most cost-effective room to retrofit–in this study, bedrooms had the most installed watts, but used only half the lighting energy of kitchens (see Figure 2). Auditors evaluating cost-effectiveness should also keep in mind the difference between usage during dark and light periods.

Figure 2. Residential lighting: installed watts and energy use, by room.

Many customers in this study were already using high-efficacy lamps. Apparently, the message has been getting through that high-use areas are the best places for such lights. Jennings broke down the usage hours by lamp type. She found that nonfluorescent (primarily incandescent) lamps were on an average of 1.8 hours per day; CFLs were on 4.4 hours per day; and standard fluorescents were on 2.8 hours per day.

From Data to Savings

Once each home’s monitoring was completed, the energy auditors offered the participating customers CFLs: 5 lamps for 6 months of participation and 10 lamps for 12 months. These incentive lamps ranged from 15W to 30W. The replacements were made after final measurements had been taken, and did not affect the study results.

Since the loggers were generally not reset while installed in a home, the reading at the end of the study was the total run time. The auditors looked at the last logger readings with the customers and helped customers decide where to install their incentive lamps. About 5% of the loggers had readings that did not reflect the total hours of use of the lamp, but those loggers had been flagged earlier, so they did not have much effect on the placement of the CFLs.

The auditors found that when they presented run time and installed watts data to customers, the efficient lamps were installed in the fixtures with the highest hours of use 66% of the time. They were installed in the highest watt-hour fixtures 77% of the time. Customers occasionally chose to install incentive lamps in less-used fixtures because they were dissatisfied with fit, color, or brightness in the more-used fixtures. Sometimes the CFLs the customers chose to install were of higher wattage than recommended because they wanted more light than they had had before.

Table 2. Recommended Wattage of Replacement Lamps
Incandescent CFL Watts Light
Watts Output Change
25 7 104%
40 11 110%­120%
60 15 105%
75 20 102%
100 25 102%
Source: Philips Lighting Company

Table 2 shows the lamp wattages that are widely recommended for retrofitting incandescent lamps with CFLs. These numbers are not universally accepted, however. According to LRC’s 1993 Specifier Reports: Screwbase Compact Fluorescent Lamps, replacing incandescent lamps with CFLs of one-fourth the wattage is inadequate. LRC recommends using CFLs of one-third the wattage of incandescent lamps, because of differences in the nature of illumination provided.

Table 3 shows the projected savings when customers replace lamps. It is unlikely that any program would be successful in replacing all of the eligible lamps, but this indicates the maximum potential savings.

The complete data set from the study is available at the Energy Services Office of Tacoma Public Utilities for use by energy conservation researchers. To obtain a copy, call David Lerman at (206)502-8619.

Table 3. Projected Potential Savings from Replacing Incandescent Lamps with CFLs.1
On >=1 On >=2 On >=3 On >=4
Hour/Day Hours/Day Hours/Day Hours/Day
Annualized savings per lamp (kWh) 77 106 140 165
Local rate of 4¢/kWh Annual savings per lamp $3.08 $4.24 $5.60 $6.60
Simple payback2 (yrs) 4.9 3.5 2.7 2.3
Local rate of 8¢/kWh Annual savings per lamp $6.16 $8.48 $11.20 $13.20
Simple payback2 (yrs) 2.4 1.8 1.3 1.1
Local rate of 14¢/kWh Annual savings per lamp $10.78 $14.84 $19.60 $23.10
Simple payback2 (yrs) 1.4 1.0 .77 .65
Number of lamps, studywide 2,047 1,285 797 590
1. Assumes 50W-59W incandescents will be replaced by 15W CFLs; 60W-75W by 20W; 76W-100W by 27W; and 101W-150W by 30W.
2. Assumes initial cost of $15 per lamp for CFL.

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