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.
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.
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.
Develop a draft layout and identify potential fixtures. Even if the fixtures have not been finalized, develop calculations (see below) for the lighting power allowance and installed lighting power for the project to achieve at least a 15% reduction from the allowance. This provides a realistic target for your lighting designer to work toward.
