Shedding Light on Home Lighting Use

Lyle Tribwell is currently working on community based energy efficiency projects in conjunction with the Energy Outreach Center in Olympia, Washington.

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

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?

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.

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.

Loggers were installed using aluminum-clad fiber-optic extensions such as the one clearly visible here. The extensions keep the loggers from being affected by light from the sun or other lamps, and allow them to be mounted far enough from the lamps to prevent melting.

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.

• 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.

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.

The author demonstrates the installation of a lighting logger on an outdoor fixture. Program participants not only agreed to have loggers installed, but also allowed auditors access to their homes monthly to read and check the loggers.

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.

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 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.
 
 

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