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.
This method is easier to document and may be more appropriate if your project includes only a few space types, as it references a single LPD for the whole building.
Do these calculations early in the design stage so that the owner and MEP can get a sense of what overall energy consumption will be and whether this LEED credit is achievable.
The space-by-space method might be better if you have lots of display or decorative lighting. It allows you to take credit for lighting power optimization for each space.
Use the building-area method if you want to focus on total project lighting power. In this case, you don’t calculate the lighting power of individual spaces.
Exempt fixtures include emergency lighting, lighting within dwelling units, and lighting required by health and safety statutes. Certain display lighting is also exempt from the calculations.
Lighting power allowance, in watts, is the limit of total wattage for a building or space type allowed by ASHRAE 90.1-2007. It is derived by multiplying the LPD by the gross lighted floor area.
