This method requires that the installed lighting power for each space be calculated individually. The installed lighting power (in watts) of each space type is then compared to the allowable lighting power (in watts) to determine the percentage lighting power reduction.
To determine the total installed lighting power (in watts), add up the total watts for all lighting in the project building (omitting exempt fixtures, as noted above). This is your installed lighting power.
To determine the lighting power allowance (in watts) for your project building, look up the building type that best represents your project in Table 9.5.1 of ASHRAE 90.1-2007. Multiply the allowable LPD for that building type (in watts/ft2) by the gross lighted floor area of the project. This is your lighting power allowance.
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.
Decide which calculation method you’ll use (see below for more). If the choice isn’t clear, run the numbers both ways and see whether the building-area method or the space-by-space method works out better.
