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.

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. In general, the LPD requirements are less stringent than the building area method, and increases to lighting power allowances are allowed for decorative appearance lighting and for highlighting merchandise. However, it’s more work to document the space-by-space method. 

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.

In California you can use Title 24 instead of ASHRAE 90.1 as your standard for credit compliance and documentation.

Refer to the ASHRAE tables to figure out your lighting power allowances by building and space type (tables 9.5.1 for building type and 9.6.1 for space type). For each building or space type there is a corresponding LPD in the table. To calculate the lighting power allowance for your project or space type, multiply the LPD by the gross lighted floor area.

LPD has no direct relationship with how much light the fixtures
provide. Good lighting design focuses on footcandle (fc) levels on the
work surface (30” above finished floor, in the case of an office). A
typical office design has light levels of 30–50 fc. Contrast and
ceiling and wall brightness should also be considered part of good
lighting design as they can have a significant impact on the actual and
perceived light levels in the space.

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. 

Lighting power density (LPD) is defined as the total amount of lighting power, in watts, provided to a given floor area, in square feet. A conventional general office lighting design has an LPD around 1.2 W/ft2, while an optimized design can be 0.8 W/ft2 or below. 

For EAp2: Minimum Energy Performance, you have to hit a target of 10% lower LPD than the ASHRAE 90.1-2007 lighting allowance. For this credit, you can earn points starting at 15% reduction for one point, and in 5% increments up to 35% for 5 points (plus an Exemplary Performance point at 40%, via IDc1).