Why LPD decides whether your project gets signed off
A lighting design has to satisfy two different constraints. Illuminance targets describe what people need to see; lighting power density describes what the building is allowed to burn. You can hit every lux target on the drawings and still fail the energy code, because the two are not the same test.
LPD is a watt budget per unit of floor area — W/ft² in US codes, W/m² elsewhere (1 W/ft² = 10.76 W/m²). Exceed the allowance and the fix is not a redesign conversation: it is fewer watts, more lumens per watt, or allowances you forgot to claim. On a live site that means pulling fixtures, adding controls that were never in scope, and holding up the certificate of occupancy until it is resolved.
What LPD actually measures
The arithmetic is simple:
LPD = total connected lighting load (W) ÷ floor area (ft² or m²)
Lighting Power Allowance (LPA) = floor area × LPD limit
The arguments are all about the numerator.
Use input watts, not "LED watts." The figure in the compliance spreadsheet is the luminaire's rated input power — LED, driver and any integral control device. A fixture sold as "40 W" with a mediocre driver can draw 46 W at the terminals, and that gap is enough to fail a tight budget. Ask for the LM-79 report, not the catalogue headline.
Parasitic load counts. DALI bus supplies, wireless nodes, occupancy sensors and driver standby draw all sit inside the connected load.
Some lighting is exempt. Under ASHRAE 90.1 Section 9.1.1, emergency egress lighting and exit signage are excluded, as are certain display, decorative and process applications. An exemption you cannot document is not an exemption — put exempt circuits on their own schedule.
Three compliance paths
ASHRAE/IES 90.1-2022 offers three prescriptive routes. All three require you to clear the mandatory control provisions of Section 9.4 first; the watt budget is the second gate, not the first.
Simplified Building Method (Section 9.3). For office buildings the total interior LPA must not exceed 0.56 W/ft² (6.0 W/m²), with garage floors capped at 0.14 W/ft². In exchange you accept a fixed control package: automatic shutoff, a manual control able to cut lighting by at least 50%, and manual-ON occupancy sensing in enclosed spaces.
Building Area Method (Section 9.5.1). One LPD value for the whole building, by building type. Easy to document, but a lobby and a storage room get the same allowance.
Space-by-Space Method (Section 9.5.2). A separate allowance per space type, so you spend watts where the visual task is demanding and save them in corridors and storage. This is what most designers use, and it is the only path that unlocks additional allowances.
The numbers: what the 2022 edition allows
Building Area Method limits in ASHRAE/IES 90.1-2022 (metric values rounded):
| Building area type | LPD (W/ft²) | LPD (W/m²) |
|---|---|---|
| Office | 0.62 | 6.7 |
| Retail | 0.78 | 8.4 |
| School / university | 0.70 | 7.5 |
| Hospital | 0.92 | 9.9 |
| Health care clinic | 0.77 | 8.3 |
| Manufacturing facility | 0.82 | 8.8 |
| Warehouse | 0.45 | 4.8 |
| Hotel / motel | 0.53 | 5.7 |
| Transportation | 0.56 | 6.0 |
| Parking garage | 0.17 | 1.8 |
The Space-by-Space table is much longer; these sample values show what it gives you in task-heavy rooms:
| Space type | LPD (W/ft²) | LPD (W/m²) |
|---|---|---|
| Office, enclosed ≤150 ft² | 0.74 | 8.0 |
| Office, enclosed >150–300 ft² | 0.67 | 7.2 |
| Office, open plan >300 ft² | 0.56 | 6.0 |
| Restroom | 0.75 | 8.1 |
| Stairwell | 0.47 | 5.1 |
| Storage room ≥50 ft² | 0.36 | 3.9 |
| Parking garage, general | 0.11 | 1.2 |
Two trends matter: interior LPD values fell roughly 4% from 90.1-2019 to 90.1-2022, and the enclosed-office size breakpoint shifted from 250 ft² to 150 ft², so an office that once qualified for the small-office allowance may no longer. Which edition applies is decided by your Authority Having Jurisdiction, not by publication date — several US states still enforce 2016 or 2019, and California runs its own code.
Additional allowances worth claiming
Section 9.5.2.2 lets you add power for specific functions instead of fighting over the base allowance: decorative lighting at 0.70 W/ft² (7.5 W/m²), retail display tiered from 0.40 to 1.00 W/ft², and a new videoconferencing allowance of 0.50 W/ft² (5.4 W/m²) where the space meets ANSI/IES/AVIXA RP-38-17. That last one is worth checking on every meeting room rebuilt since 2020.
The control package that travels with the watt budget
Automatic shutoff. Interior lighting in buildings over 5,000 ft² must shut off automatically — on a time schedule, by occupant sensor within 30 minutes of the space emptying, or from a signal from another system. Spaces required to run 24 hours and spaces where patient care is delivered are exempt.
Continuous dimming replaced bi-level switching. 90.1-2022 removed the old intermediate step between 30% and 70% power; a manual dimmer providing continuous dimming down to 10% or less is now required. Confirm the driver holds stable output at the bottom of the curve — 10% is a real operating point, not a footnote.
Daylight-responsive controls kick in much earlier. The threshold dropped to 75 W of connected general lighting in the primary sidelighted zone and 150 W in the secondary zone. At 40 W per panel, 75 W is barely two fixtures.
Open offices over 300 ft² changed most. They now need occupancy controls with zones limited to 600 ft². Lighting in the occupied zone may come on to full; lighting elsewhere may also come on, but to no more than 20% of full power. That is a wiring and grouping requirement, not something to solve at commissioning.
Worked example: a 2,000 m² open-plan office
Fixture A is a 600×1200 backlit LED panel, 40 W input, 4,800 lm (120 lm/W). Fixture B is the same format, 40 W input, 4,000 lm (100 lm/W). Using the lumen method with a utilisation factor of 0.75 and a maintenance factor of 0.80:
- Fixture A: 400 × 2,000 ÷ (4,800 × 0.75 × 0.80) = 278 fixtures → 11,120 W → 0.52 W/ft²
- Fixture B: 400 × 2,000 ÷ (4,000 × 0.75 × 0.80) = 334 fixtures → 13,360 W → 0.62 W/ft²
Against Building Area Method (0.62 W/ft², allowance 13,347 W), Fixture A passes with about 17% headroom; Fixture B is 13 W over — a fail on paper. Against Space-by-Space (0.56 W/ft², allowance 12,142 W), Fixture A clears it with roughly 8% headroom; Fixture B is 1,218 W over, a fail by 10%.
Same room, same layout logic, same catalogue wattage. The only difference is 20 lm/W of system efficacy, and it moves the project from comfortable compliance to a rejected submission.
Where projects actually fail
Picking a round wattage and spacing fixtures until the room looks right, without running the connected load until submission week. Designing to catalogue lumens rather than delivered system efficacy — a 130 lm/W LED behind a diffuser that delivers 105 lm/W is a 105 lm/W luminaire. Ignoring driver efficiency and standby draw. And discovering the control requirements after the ceiling is closed.
What LPD means for how you buy
Once you accept the code is a watt budget rather than a brightness target, the purchasing logic shifts. System efficacy at the terminals becomes a first-tier specification; the dimming interface and its low-end behaviour become a code item; and the LM-79 report, LM-80/TM-21 data and DLC listing stop being paperwork and become evidence that a fixture will hold performance long enough to stay compliant.
That comes down to five questions on every RFQ: what is the measured input wattage; what is the luminaire efficacy in lm/W; does the driver dim continuously and stably to 10%; what is the standby power of the driver and any control node; and will the supplier provide a space-by-space LPD calculation for this project.
A short closing note
LPD is not the most interesting number in a lighting design, but it is the one that decides whether the project closes. Treat it as a budget to allocate deliberately: spend watts where people need light, save them where they do not, claim the allowances you are entitled to, and lock the control package in before the layout is frozen.
Solarix Lighting builds commercial LED panels, downlights, cleanroom luminaires and street lights for this kind of work, and we supply the LM-79 data, DLC documentation and dimming specifications a code submission needs. Send us the space list and the adopted edition, and we will run the LPD calculation with you before you order.