On a small tenant fit-out, a fixture’s power factor or total harmonic distortion can seem like minor cut-sheet details. On a project with many LED luminaires, they are part of the electrical design conversation. A luminaire can meet a target lumen output and wattage while still drawing more current than its real wattage suggests or adding current-waveform distortion to a circuit.
That does not make one published number a universal pass/fail test. Requirements vary by jurisdiction, utility program, project electrical criteria, and the operating mode being submitted. The practical goal is to ask for complete, configuration-specific driver data and have the electrical engineer evaluate it in the context of the branch circuit, panel, controls, and aggregate load.
Start with the two measurements
Power factor (PF) compares real power in watts with apparent power in volt-amperes (VA). The U.S. Department of Energy describes power-quality metrics in those terms: real power is the active power consumed, while apparent power reflects volts and amps together. With the same real wattage, a lower-PF luminaire draws more current than a higher-PF luminaire. ENERGY STAR makes the same current-draw distinction in its downlight criteria.
Total harmonic distortion (THD) describes waveform distortion. In a lighting submittal, ask whether the value is current THD (often written THD-I or THDi), not simply an unlabeled “THD” figure. A driver may use power-factor-correction circuitry to support high PF and low THD, but they are not interchangeable metrics: PF is a power relationship, while THD is a distortion measurement. The DOE’s LED-driver research identifies power-factor correction as a distinct driver function intended to promote high PF and low THD.
Neither value measures useful light. Continue to review delivered lumens, input watts, distribution, color, controls, thermal conditions, and photometry for the exact proposed configuration. The LM-79 and IES-file review process is still the right way to confirm measured light output and the distribution used in calculations.
Why the exact operating condition matters
LED luminaires commonly have selectable outputs, dimming, occupancy response, photocells, networked controls, or multiple voltage options. Driver input behavior can change by load and operating mode. A catalog-family statement is useful context, but it is not necessarily proof for every selected configuration.
The DesignLights Consortium’s SSL Technical Requirements Tables V5.1 require qualified luminaires to meet PF of at least 0.9 and THD of no more than 20% at their worst-case loading conditions. That is a useful benchmark for product classes covered by that program, not a substitute for a project’s own criteria. The current DLC table also makes the worst-case condition explicit.
ENERGY STAR criteria illustrate why a label alone does not establish the same threshold for every lighting category. Its downlight criteria set PF at 0.7 or higher, whereas commercial and industrial products in the DLC scope are subject to the stricter 0.9 / 20% benchmark noted above. Read the applicable program and product category rather than assuming every fixture certification implies the same electrical-performance limit.
For broad commercial and industrial procurement, the DOE Federal Energy Management Program points specifiers to DLC criteria for troffers, linear ambient fixtures, high bays, and low bays, and notes that those technical requirements address PF and THD alongside light quality and lumen maintenance. This makes the values particularly relevant when comparing alternatives in those categories.
Write the requirement so it can be verified
If the construction documents need a threshold, make the condition testable. “High power factor” and “low harmonics” are too vague to check in a submittal. Instead, coordinate a requirement with the electrical engineer that identifies the measurement and the intended condition.
| Ask for | Why it belongs in the requirement |
|---|---|
| Minimum PF | States the real-power-to-apparent-power criterion instead of relying on a marketing adjective. |
| Maximum THD-I / THDi | Clarifies that the limit concerns input-current distortion and gives the reviewer a comparable quantity. |
| Input voltage and frequency | Prevents a 120 V result from being assumed for a multi-volt submitted fixture. |
| Output setting and dimming state | Records whether the numbers apply at full output, a selected lumen package, or the least favorable published mode. |
| Test method or manufacturer documentation | Gives the reviewer a traceable source rather than an unsupported catalog claim. |
| Product configuration | Ties the driver information to the submitted optic, output, controls, and catalog number. |
For example, a project might require values at full rated output across the specified input-voltage range, then separately ask whether dimmed or reduced-output modes have different published values. Do not copy a threshold from another project without confirming its scope. Older public specifications can be informative but may not match the local code, utility, or electrical-system requirements now governing the work.
Review electrical impact with the right team
Lighting designers and specification writers can make the submittal complete, but they should not independently turn fixture PF and THD values into feeder or neutral conclusions. Aggregate nonlinear loading, circuit layout, transformer design, other electronic loads, controls, and local requirements are electrical-engineering questions.
Bring the electrical engineer in early when a project has large quantities of high-bay lights, continuous linear lighting, large troffer packages, or a site-lighting retrofit with many area lights. The useful package is a fixture schedule with quantities and circuit assumptions plus the selected product’s voltage, watts, PF, THDi, driver type, dimming method, and source documentation. The engineer can then determine whether further load or harmonic analysis is warranted.
This coordination is also important for substitutions. A proposed alternate with comparable lumens and watts can use a different driver platform. Ask for the alternate’s evidence at the same voltage, output, and control condition as the basis-of-design fixture; do not accept a generic family statement as proof of equivalence.
A concise submittal checklist
For each scheduled fixture or alternate, retain:
- Complete submitted catalog number and selected options.
- Published input voltage, frequency, wattage, PF, and explicitly labeled THD-I/THDi.
- The condition behind each number: output setting, dimming or control state, and any worst-case statement.
- Driver cut sheet, test report, or manufacturer confirmation that traces the metrics to the offered configuration.
- Applicable program evidence when DLC, ENERGY STAR, a utility, or an owner requirement is part of the project.
- Electrical-engineer review when aggregate fixture loading or project criteria call for it.
The takeaway
PF and THD are not decorative driver specifications. PF helps describe current draw for a given real load, and THD identifies current-waveform distortion; neither replaces photometric, controls, or electrical-system review. State the measurement, threshold, voltage, and operating condition in the documents, request configuration-specific evidence, and have the electrical team evaluate the combined installation. That gives the project a more defensible basis for selecting an LED luminaire than a vague “high-quality driver” claim.