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. 

To determine the space-by-space lighting power allowance for the project:

List all spaces in the building by space type designated in ASHRAE table 9.6.1.b. 
Determine the gross lighted floor area of each type of space in square feet.
From ASHRAE table 9.6.1, determine the allowable LPD for each space type. 
Multiply the area of each space by the allowable LPD. This equals the total allowed wattage for each space.
Add up all wattage for the different space types to find the lighting power allowance for the project.

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. 

Space-by-Space Method

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

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.

Pursuing the energy model option can help you document compliance with the energy prerequisite. It can also be useful in applying for financial incentives, which are usually based on a percentage reduction (energy saved) relative to your local code.

The energy modeler should be contracted during early design phase to provide recommendations for a high-performing system and equipment selection along with ideas about potential energy savings, cost estimates, and payback periods.  

Many CI projects with limited HVAC design scope find it more cost-effective to pursue Option 1. The usefulness of energy modeling as a design and optimization tool depends upon how wide the project scope is—the more systems that can be influenced in the design process, the better.