Lighting Power Density: Calculation, Methods, and COMcheck

Lighting power density is the total connected lighting wattage of a building divided by its interior floor area, expressed in watts per square foot. Both the International Energy Conservation Code and ASHRAE/IES Standard 90.1 cap this number for commercial buildings, and a design that exceeds the cap will not pass plan review. The allowable value depends on your building type, the compliance method you choose, and which edition of the energy code your jurisdiction has adopted. Allowances in recent editions run from as low as 0.11 W/ft² for an interior parking area up to roughly 0.92 W/ft² for a hospital.¹ Before you design to any specific number, confirm the code edition in force where the project is being built.

How the Calculation Works

The formula is simple division. The complication sits in what you count.

In the numerator, “total connected lighting wattage” is not just the lamps. It includes every component that draws power to produce or regulate light: lamps, ballasts, LED drivers, current-limiting devices, and transformers. Track lighting is counted at either the maximum rated wattage of the track or the total wattage of the fixtures attached to it, whichever the code calls for.¹

In the denominator, floor area is the horizontal area measured from the inside surface of the exterior walls. When you are working out net occupiable area for a specific space, shafts, column enclosures, and permanently enclosed areas that nobody occupies come out of the total.² A few hundred square feet of error compounds quickly on a borderline design, so verify dimensions against the architectural drawings before you run the compliance numbers.

Lighting That Does Not Count

Several categories of lighting sit outside the energy code’s lighting requirements entirely and should be stripped out before you add up wattage:

Emergency fixtures that stay off during normal operation and only activate during a power failure.
Exit signs and any illumination specifically required by health or life-safety regulations.
Decorative gas lighting.
Public roadway and traffic lighting, including driveway entrances within the public right-of-way.³

Leaving exempt fixtures in your tally inflates the installed wattage and can turn a compliant design into a failed one on paper.

Which Code and Edition Apply

Two model codes govern commercial lighting energy use in the United States: the IECC and ASHRAE/IES Standard 90.1. Most jurisdictions adopt one or the other, sometimes with local amendments. As of 2025, several states enforce the 2024 IECC or ASHRAE 90.1-2022, while many others are still on the 2021 IECC paired with ASHRAE 90.1-2019.⁴ A handful of states have no statewide commercial energy code at all, leaving adoption to cities or counties.

Edition matters because the numbers move. ASHRAE 90.1-2022 sets retail at 0.84 W/ft² under its Building Area table, while the IECC lists 0.78 for the same category.⁵¹ Confirm the adopted code with your local building department before designing to any value in the tables below.

Three Ways to Show Interior Compliance

ASHRAE 90.1-2022 offers three prescriptive paths: the Simplified Building Method, the Building Area Method, and the Space-by-Space Method.⁶ The IECC offers the Building Area and Space-by-Space methods.¹ Every path requires you to meet both the power limit and the lighting control rules.

Simplified Building Method

Available under ASHRAE 90.1-2022 for three building categories only: offices, retail, and schools. One LPD value covers the whole interior, with a separate value for any attached parking garage:⁵

Office: 0.56 W/ft² interior, 0.14 W/ft² parking garage.
Retail: 0.70 W/ft² interior, 0.14 W/ft² parking garage.
School: 0.63 W/ft² interior, 0.14 W/ft² parking garage.

Multiply gross lighted floor area by the applicable value for your allowance. The interior and parking garage allowances cannot be combined or traded against each other. This method fits early design, when room layouts have not been finalized and the building falls cleanly into one of the three categories.

Building Area Method

You identify the building’s primary function in the code’s table, pull the LPD value, and multiply by the total interior floor area. Representative IECC allowances:¹

Warehouse: 0.45 W/ft²
Hotel/motel: 0.53 W/ft²
Office: 0.62 W/ft²
Retail: 0.78 W/ft²
Manufacturing facility: 0.82 W/ft²
Hospital: 0.92 W/ft²

If total installed wattage comes in at or below the allowance, the design passes. This method works best for buildings with a single dominant use. Buildings that mix substantially different functions usually fit the next method better.

Space-by-Space Method

You calculate a separate allowance for every enclosed room or area, matching each to a category in the space-type table and multiplying its floor area by the corresponding LPD value. An enclosed office might carry 0.73 W/ft², a storage room 0.35, an interior parking area just 0.11.¹

Add the individual allowances together to get the building’s total interior allowance. Trade-offs between spaces are permitted so long as the total installed wattage stays within that combined number.⁵ A corridor that comes in under its allowance frees up budget for a conference room or lab that needs more light. Hospitals, universities, and mixed-use commercial buildings almost always run this method for that reason.

One rule to flag: buildings with unfinished spaces must use the Space-by-Space Method, and the allowance for any unfinished space is capped at the lesser of actual connected lighting or 0.1 W/ft².

Additional Allowance for Retail Display

Retail buildings using the Space-by-Space Method pick up supplementary lighting power on top of the base LPD, because merchandise display needs accent lighting that general illumination cannot provide. Under ASHRAE 90.1-2022, the extra allowance starts at a 750-watt base plus area-based additions that vary by product category:⁷

General merchandise: 0.40 W/ft²
Vehicles, sporting goods, small electronics: 0.40 W/ft²
Furniture, clothing, cosmetics, artwork: 0.70 W/ft²
Jewelry, crystal, china: 1.00 W/ft²

The extra power is conditional. Display lighting must be installed and controlled independently from the general lighting in the space, so you cannot fold it into overhead fixtures and claim it. Other merchandise types may qualify for a higher tier if you document the visual or contrast requirement and the local authority approves.

Exterior Lighting Is a Separate Budget

Interior and exterior allowances do not mix. Unused interior wattage cannot light your parking lot, and the reverse is also prohibited.⁵ The exterior allowance adds a base site allowance to individual area allowances, with LPD values that depend on which of ASHRAE 90.1’s five lighting zones (0 through 4) applies to the site. Zone 3, a typical commercial area, allows 0.037 W/ft² for uncovered parking, 0.18 W/ft² for entry canopies, and 0.26 W/ft² for loading docks. Zone 0, covering undeveloped areas, gets no exterior allowance at all. Façade lighting runs on its own calculation and cannot be traded with other exterior areas.

Lighting Controls Are a Separate Requirement

Meeting the LPD limit is only half the picture. Both the IECC and ASHRAE 90.1 require automatic lighting controls in most commercial spaces regardless of which power compliance method you choose. A design that passes on wattage but misses the control requirements still fails plan review.

Under ASHRAE 90.1-2022, each space type carries specific control functions. The common ones:⁸

Occupancy sensor reduction: general lighting drops by at least 50% within 15 minutes after a space is vacated.
Large offices over 300 ft²: control zones capped at 600 ft², and lighting reduces by at least 80% within 20 minutes of vacancy.
Partial automatic-on: no more than 50% of general lighting wattage turns on automatically when someone enters; the rest must be activated manually.
Occupancy sensor shutoff: full shutoff within 15 minutes of vacancy in certain spaces, with each sensor controlling no more than 5,000 ft².

Healthcare spaces adjacent to patient care areas, 24/7 continuous-lighting spaces, and spaces where shutoff would endanger occupants carry exceptions to the full-shutoff rule. The control strategy ties directly to the LPD submission because the controls appear on the same electrical drawings that accompany your compliance report.

Documenting Compliance with COMcheck

Most jurisdictions accept COMcheck, a free tool from the U.S. Department of Energy, to demonstrate compliance. The software evaluates the design against the code edition your jurisdiction has adopted and generates separate Interior Lighting and Exterior Lighting Compliance Certificates showing installed wattage against allowed wattage.⁹¹⁰

COMcheck also produces phase-by-phase inspection checklists for the permit file, and the compliance statement on the certificate must be signed before submission. Many jurisdictions also require a licensed architect or engineer to seal the construction documents, though that requirement comes from professional licensing rules rather than from COMcheck.

The compliance package goes in alongside the electrical plans and fixture schedules. Plan reviewers compare the COMcheck output against the drawings to confirm that every fixture, lamp type, and wattage listed in the software matches what the plans actually show. Mismatches between the report and the drawings are one of the most common causes of permit delays, so reconciling the COMcheck input with the fixture schedule before submission saves time.

After Approval: Field Inspection

Plan approval is not the last step. Before a certificate of occupancy, a field inspector verifies that installed lighting matches the approved documents. Fixture locations and types are checked against the as-built drawings, and all required controls are tested for proper installation and function. Occupancy sensors are reviewed for appropriate technology: passive infrared works in open areas with clear sightlines, while ultrasonic or dual-technology sensors are needed where obstructions would block infrared detection. Daylight-responsive sensors receive particular attention, since they have to be positioned where they actually read ambient light rather than activity from adjacent spaces.

When installed work diverges from the approved plans, the project is typically flagged for correction before occupancy is permitted. The gap between what gets designed on paper and what gets wired in the field is where most compliance problems surface, so the simplest safeguard is to have the electrical contractor work directly from the approved COMcheck fixture schedule and avoid field substitutions.

1
Building Energy Codes Program. Interior Lighting LPD Update
2
ASHRAE Terminology. ASHRAE Terminology – Net Occupiable Area
3
ASHRAE. ANSI/ASHRAE/IES Addendum ab to Standard 90.1-2022
4
State Climate Policy Dashboard. Commercial Energy Codes
5
ASHRAE. ANSI/ASHRAE/IES Standard 90.1-2022
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ASHRAE. Lighting Changes in ASHRAE/IES Standard 90.1-2022
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ASHRAE. ANSI/ASHRAE/IES Addendum z to Standard 90.1-2022
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ASHRAE. ANSI/ASHRAE/IES Addendum bd to Standard 90.1-2022
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Building Energy Codes Program. Legacy COMcheck
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Building Energy Codes Program. COMcheck Basics