Estimating Energy Savings

As energy-saving automatic lighting controls become more popular for retrofit options in existing buildings, electrical contractors may find themselves in a position of estimating energy savings to justify owner investment.

In existing buildings, cost-effective retrofit options are available. The biggest challenge is adding low-­voltage control wiring, generally limiting opportunities for sophisticated control systems. As a result, the simplest upgrade options impose the least amount of rewiring, or they involve swapping out older controls and ballasts for new ones.

Examples include upgrading control panels to intelligent panelboards with scheduling capability, replacing toggle switches with wall-switch occupancy sensors or timer switches, and replacing fixed-output fluorescent ballasts with line-voltage dimmable ballasts. Radio-frequency (RF) wireless controls enable photosensors, occupancy sensors and new switches to be added anywhere within radio range. 

If new luminaires are installed, they can be specified with onboard controllers and sensors—useful with light-emitting diode (LED) luminaires, which often add negligible costs for dimming. These options may be a standalone upgrade or be incorporated into a lamp/ballast retrofit. The trouble is that, while estimating energy savings from replacing lamps and ballasts is fairly straightforward (based on differences in system input watts), lighting control savings are often variable, based on the application. Occupant behavior, building design, site orientation, daylight availability, device settings and level of commissioning can affect actual energy savings.

Energy savings for scheduled automatic shutoff, in which lights are turned off at a certain time of day, and institutional task tuning, in which the facility operator establishes a fixed maximum light/power level for each space, may be fairly easy to predict as the outcomes are somewhat rigid. Other strategies, however, may prove problematic.

A survey of ECs and energy consultants conducted by the Lighting Controls Association found that about 40 percent of respondents typically base control energy savings estimates on personal experience or an evaluation of the application. There are other ways that can supplement personal experience, including trial installations, manufacturers, benchmarks, industry research and energy codes.

The trial installation is perhaps the most accurate method. It involves a partial installation in a typical space to generate data suggestive of typical savings. If occupancy sensors will be installed, light loggers can also be implemented. These devices record the length of time lights are left on while the space is unoccupied. However, this path is not always available; just 8 percent of survey respondents said they most often base energy savings estimates on trial installations.

Manufacturers are another great source of help. They offer experience, intimate knowledge of their products, and, in some cases, data for monitored projects. However, their estimates and case studies are likely to be based on systems designed to produce optimal results. Twenty-one percent of the survey respondents most often use this method.

Benchmarks—energy savings generated in projects similar to the one under consideration—can be helpful. These can be collected through networking with colleagues and reviewing case studies.

Studies by organizations such as the California Lighting Technology Center and the Lighting Research Center can be a valuable guide. One of the most useful is a 2011 study by the Lawrence Berkeley National Laboratory, which conducted an extensive review of prior research to produce best estimates of lighting energy savings for occupancy sensors (24 percent), personal tuning (31 percent), daylight harvesting (28 percent), institutional tuning (36 percent), and any combination of the above (38 percent). These studies are credible because they were produced using a professional research method. Eighteen percent of the Lighting Controls Association survey respondents favor this method.

Finally, energy codes can prove useful while also posing a high degree of credibility as coming from a respected authority. Both the ASHRAE/IES 90.1 2010 energy standard (Table 9.6.2) and the California Title 24 2013 energy code (Table 140.6-A) list advanced control options with power adjustment credits; 90.1 cross-references these control strategies with common applications. For example, if a multiscene programmable dimming system is installed in a restaurant, a power adjustment credit of 0.20 is available. These credits suggest of typical energy savings, in this case, 20 percent.

Note that the energy savings potential for lighting controls is best realized if the equipment is properly commissioned. At a minimum, this includes a written control narrative describing functionality, verification that controls are properly installed, and testing. A 2012 Energy Center of Wisconsin study found that, after recommissioning daylight harvesting lighting controls in 20 office and public spaces, median lighting and associated HVAC energy savings approximately doubled from 23 to 43 percent.

Estimating lighting control energy savings poses a greater challenge than with lamp/ballast systems and luminaires, but resources are available to help produce estimates that the owner can count on.

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