What is lighting power density?

Lighting power density, or LPD, is the installed lighting wattage of a space divided by its floor area, expressed in watts per square foot. Energy codes cap it so buildings do not install more lighting power than necessary, which directly limits lighting energy use.

Which ASHRAE allowances does this use?

It uses the ASHRAE 90.1-2022 building-area method allowances from Table 9.5.1, such as 0.64 W/ft² for offices and 0.84 for retail. These are whole-building averages; the space-by-space method in Table 9.6.1 sometimes permits more in specific room types.

Should I use installed or connected wattage?

Use the installed power of the luminaires — the total input wattage of each fixture including drivers and ballasts, multiplied by quantity. For LED fixtures use the rated input watts, not the bare LED-board wattage, so the figure reflects what the panel actually serves.

Are any lights exempt from the LPD limit?

Yes. ASHRAE 90.1 Section 9.1 exempts certain lighting such as specialized medical, theatrical, and some display and emergency lighting. Exclude exempt fixtures from the total so they do not inflate the installed density and cause a false failure.

Does passing this guarantee code compliance?

No. LPD is one requirement among several — controls, daylighting, and exterior lighting also apply. The adopted energy code may be the IECC rather than ASHRAE 90.1, and editions and local amendments differ, so confirm the version in force with the authority having jurisdiction.

Lighting Power Density (LPD) Calculator

Keeping lighting under the energy budget

Energy codes limit how much lighting power a building may install, measured as watts per square foot. Stay under the limit and the lighting design passes; exceed it and the plan check bounces back. This calculator totals the installed fixture wattage for a space, divides by the floor area, and compares the result against the ASHRAE 90.1 allowance for the building type.

How it works

The formula is straightforward:

LPD (W/ft²) = total installed lighting power (W) / floor area (ft²)

The total installed power is the sum over every fixture type of quantity times input watts each. The tool then looks up the building-area-method allowance for the selected occupancy from ASHRAE 90.1-2022 Table 9.5.1 and compares. It also reports the allowed power — allowance times area — and the remaining wattage budget, so you can see how much headroom is left before adding fixtures.

Example and notes

A 2000 sq ft office with twenty 32-watt troffers and ten 12-watt downlights draws 760 W installed, or 0.38 W/ft² — comfortably under the 0.64 W/ft² office allowance, leaving roughly 520 W of budget. Use rated luminaire input wattage (including drivers), exclude any lighting exempt under Section 9.1, and remember that the space-by-space method may permit a higher density in specific rooms. Always confirm whether ASHRAE 90.1 or the IECC is the adopted code and which edition applies.