Test strategies to reduce lighting power density

Example of indirect lighting design strategy. Credit: Finelite. NREL 25515

While it is not common that a building energy model is developed solely for the purpose of evaluating electric lighting strategies, BEM can provide useful information about both the direct lighting energy savings and the impact that changes in lighting power have on heating and cooling loads and the associated HVAC energy consumption.

When lighting power is represented in a building energy model, it is typically entered in terms of lighting power density, which is the installed lighting power normalized per unit of floor area, typically watts per square foot or watts per square meter.

Impact of strategies to reduce lighting power density

Direct electric lighting energy

When lighting power changes, the direct impact is, of course, on the electric energy consumed by lighting equipment and the associated impacts on peak electric demand, energy cost and CO₂ emissions.

Internal heat gain

A change in lighting power results in a change in internal heat gain. An important point to understand is that 100 percent of electric energy consumed by interior lighting fixtures ends up as heat within the building. The location of that heat gain depends on the configuration of the lighting system. For a suspended lighting system, all of the heat ends up within the space. For a recessed lighting system, a portion of the heat may go to the space above the ceiling. For more information about the nature of heat gain from electric lights, see basics of internal heat gains.

Heating and cooling energy

Due to the impact on internal heat gain, varying lighting power affects space heating and cooling loads and the associated HVAC energy consumption. When lighting power and its associated heat gain is reduced, the lower internal heat gain results in reduced cooling energy and increased heating energy.

These indirect impacts can be significant and will vary depending on specifics of the project, such as its climate, building envelope performance, the nature of other internal heat gains, and the efficiency of the HVAC system. However, in almost all cases when lighting power is reduced, the savings in lighting and cooling energy will be greater than increase in heating energy, providing a net reduction in energy consumption.

Cooling system sizing

The peak cooling load for a space will be affected by heat gain from electric lights. The reduced heat gain due to lower lighting power may allow selection of smaller HVAC equipment.

Alternatives

Guidance on efficient lighting design is beyond the scope of this guide, but in general, installed lighting power can be minimized in three ways: efficient lighting design strategies, efficient light sources and efficient lighting fixtures. In addition, lighting energy can be minimized with efficient controls, as discussed here: analyze the impact of lighting control strategies. Energy conservation measures related to lighting are also listed here: energy conservation measures.

Efficient lighting design strategies

Task-ambient lighting design strategy provides lower levels of general ambient illumination supplemented by task lights where needed. Credit: photo by Sheila Hayter. NREL 27461

An efficient lighting strategy provides adequate visual acuity and visual comfort by providing appropriate quantity of light (illuminance) where it is needed while also minimizing discomfort glare. Efficient strategies will typically provide only the illuminance (footcandles or lux) required based on the usage of each space. Appropriate illuminance might range from 5 footcandles in a corridor, to 40 footcandles in an office, to 100 footcandles for fine tasks.

One common strategy is task/ambient lighting design, which minimizes lighting energy consumption by providing a lower level of general light and using task lights to provide additional light where it is needed.

Efficient light sources

An efficient light source provides maximum light output (lumens) with minimum input of electric power. Light sources can be compared in terms of their efficacy (lumens/watt). LED lamps are common in new construction and can offer good efficacy, but it is important to understand that there is also a significant range in efficacy among LEDs. In other words, not all LEDs are equal.

Efficient lighting fixtures (luminaires)

The lighting fixture, the housing that contains lamps and directs their output, affects the overall lighting system efficacy. While this is less of an issue with fixtures that house LED lamps compared to larger fluorescent lamps, there can still be a significant amount of light produced by the lamps that does not escape the fixture.

Guidance on modeling approach

Primary energy model inputs related to lighting include the installed lighting power, typically entered as lighting power density, and the lighting operation schedule. In addition, some controls, such as automatic daylighting control, can also be directly represented in energy models and are discussed here: analyze the impact of lighting control strategies.

Lighting power density

The approach to determining the appropriate lighting power density input for each zone depends on whether or not a preliminary lighting design is available.

If a preliminary lighting design exists, then it will be possible to review the plans and perform a lighting takeoff to count the number of each type of lighting fixture in each space. With these fixture counts together with the input power for each fixture type as listed in the lighting fixture schedule (or manufacturer specifications), the lighting power for each space can be calculated. To save time, consider calculating the lighting power densities for a sample of different space types and use those values as input for other similar spaces. Examples of lighting plans and fixture schedules are included here: lighting system inputs.

Alternatives to the preliminary design’s lighting power density could be developed considering substitution of more efficient lighting fixtures or modifications to the proposed fixture layout.

Allowed lighting power for office buildings in ASHRAE Standard 90.1 from 1975 to 2022

If no lighting design yet exists, then assumptions will be necessary to set a baseline lighting power and more efficient alternatives. A good place to start as a baseline is the allowed lighting power in the relevant energy code. See determine the applicable energy code. If the space layout has already been developed, then consider using the relevant space-by-space lighting power allowances for each modeled space. If floorplans are not yet developed, then it may be reasonable to use the relevant whole-building lighting power allowance.

The BEMcyclopedia Lookup Tables are a good source of information about lighting power densities for a range of energy code vintages.

Efficient alternatives to the baseline lighting power could simply be defined as percentage improvements, e.g. 10% below or 30% below the allowed lighting power. Another option is to review reference sources such as those listed in sources for default internal gain assumptions for examples of efficient lighting designs.

Lighting schedules

For guidance on choosing lighting schedules, see the topic defining internal loads. The magnitude of energy savings will vary depending on the schedules used in the model, so choose schedules that are a reasonable match for expected operation to ensure that savings estimates are relevant. Schedules should reflect expected lighting control operation.

See the BEMcyclopedia Lookup Tables for examples of lighting schedules for a range of building types and space types.

Example information from an BEM output report that can be used to check that lighting power density was entered as expected

Model quality check

A model quality check can help identify model input errors.

• Check output reports that summarize zone inputs to verify lighting power.
• Check end use results for expected changes in lighting energy consumption and associated changes in heating and cooling energy.
• Check HVAC sizing changes with changing lighting power.

See also review and analysis to verify model quality.

Other considerations

Light heat to space. As noted earlier, the location within the building where the lighting heat goes can vary depending on the type of lighting fixture. When comparing lighting design strategies that use different fixture types, such as one strategy that uses pendant fixtures and another that uses recessed fixtures, this difference can affect HVAC energy consumption and is worth being considered. Many BEM programs include an input that allows the user to specify the fraction of the lighting energy that goes into the space, with the remainder typically going to a plenum zone or return air. This section, basics of internal gains, provides some relevant guidance.  

Guidance on presenting results

In addition to presenting energy results and related performance metrics that are of interest to the owner and design team, include information about the impact on HVAC system sizing.

Additional resources

• BEMcyclopedia Lookup Tables
• Illuminating Engineering Society, lighting application standards