Schematic Design

In spaces that are used primarily during the daylight hours, early design direction to provide daylight harvesting and to select high-efficacy fixtures can decrease the number of light fixtures. This can lead to significant reductions in installed lighting power, often in the range of 15%–20%. 

Lamps that are high in power density are halogen and incandescent—avoid these types of fixtures to lower your overall installed lighting power. Instead use high-efficacy lamps such as LEDs, T8 and T5 fluorescents, and compact fluorescents.

Designing for reduced ambient light and more task lighting can also reduce the number of ambient lighting fixtures you need. These tend to be more expensive than task lights, so you might have a lower first cost as a result.

Lighting power can be reduced by designing for indirect lighting with lower ambient light levels and increased task lighting. 

If you are developing an energy model for compliance with EAc1.3: Optimize Energy Performance—HVAC, it can help with the lighting system design and to optimize integration with other building systems. 

Task lights can be exempt from the calculations if they are furniture-mounted with independent controls (automatic shut-off by timer or occupancy-sensor, see ASHRAE 90.1-2007, 9.2.2.3,p). Plug-in task lights can also be exempt if they are not part of the overall lighting strategy, meaning they are not specified in the design documents. However, if they are exempted from the calculation, they cannot be counted for EQc6.1: Controllability of Systems—Lighting.

When two fixtures are provided to supplement each other, you have to assume that both fixtures are on. For example, task lights may be intended to be used when ceiling lights are turned off, but for installed wattage calculation purposes, all lights are assumed to be switched on. 

Both methods

To determine the lighting power reduction, divide your installed lighting power by the lighting power allowance.

Determine installed lighting power: Add together the wattage of the fixtures in each of the space types as listed above, except for the increased lighting power allowances for decorative or merchandise display lighting, which will need to be accounted for in a separate calculation. Be sure to exclude exempt fixtures. Sum all the wattage from each space type in the project to obtain installed lighting power.