Power Factor and THD in LED Grow Lights: What the Numbers Cost You
Two figures on a driver datasheet decide how much current an installation actually pulls, how hot the neutral conductor runs, and how much capacity you have to buy from the utility. Neither of them has anything to do with how much light reaches the canopy.
Real power, apparent power, and the gap between them
A fixture rated at 600 W converts 600 W of real power. The current it draws, however, is not simply 600 W divided by the supply voltage, because the current waveform is not a scaled copy of the voltage waveform. The product of RMS voltage and RMS current gives apparent power, measured in volt-amperes, and power factor is the ratio of the two:
PF = P / S, where P is real power in watts and S is apparent power in volt-amperes.
A 600 W fixture running at a power factor of 0.60 draws 1000 VA. The meter records 600 W, but every cable, breaker, transformer and generator upstream of it must be sized for the current that 1000 VA implies. That is the whole practical significance of the number: it is a capacity tax, not an efficiency rating.
Where total harmonic distortion fits in
Total harmonic distortion of current, usually written THDi, is the RMS sum of the harmonic currents divided by the fundamental current. A driver with a capacitor-input front end draws current in short pulses near the voltage peaks, and those pulses contain a large harmonic content. The two figures are linked by a relationship worth remembering:
PF ≈ cosφ / √(1 + THD²)
Set displacement factor cosφ to 1 and THD to 100 percent, and the true power factor falls to 0.71 even though the current is perfectly in phase with the voltage. This is why a fixture can look benign on a phase-angle argument and still draw nearly 1.41 times the fundamental current: at 100 percent THD the RMS current is √2 times the fundamental, and the resistive losses in every conductor on the way rise by about a factor of two.
Passive and active correction compared
Almost every LED driver in horticultural use falls into one of two groups. Passive correction is a choke and capacitors; active correction is a switching stage that shapes the input current to follow the voltage.
| Dimension | Passive PFC | Active PFC |
|---|---|---|
| Power factor at full load | 0.50 to 0.70 | 0.90 to 0.99 |
| Current THD at full load | Often 30 to 120 percent | Typically below 10 to 15 percent |
| Apparent power for a 600 W fixture | 860 to 1200 VA | 606 to 667 VA |
| Input current waveform | Short pulses near voltage peaks | Close to sinusoidal |
| Inrush behaviour | High, dominated by the input capacitor | Usually controlled by an inrush limiter |
| Internal losses | Low in the correction stage | A few watts consumed by the correction stage |
| Cost and weight | Lower, heavier choke | Higher, lighter |
| Typical use | Low-wattage lamps, cost-sensitive modules | Bars, panels and high-bay fixtures from roughly 150 W upward |
Neither column is a moral position. A 15 W lamp with passive correction is a sensible product; a 600 W fixture with the same front end forces the customer to buy distribution capacity that produces no light.
What the difference costs on a real array
Take a room with 40 fixtures of 600 W, a total of 24 kW of real power. At a power factor of 0.95 the array presents 25.3 kVA. At 0.60 it presents 40 kVA, a difference of 14.7 kVA that buys nothing. Generators are normally specified with a margin of about 25 percent, so the same room needs either a 32 kVA machine or a 50 kVA machine depending on a number most buyers never read.
The same arithmetic governs transformer size, cable cross-section, breaker rating and the demand charge on a tariff that bills apparent power. In a small home installation the difference is invisible. Across a multi-room facility it is often the largest single electrical decision made during procurement.
Harmonics do not stay in the phase conductor
In a three-phase four-wire system, triplen harmonics, meaning the third, ninth and fifteenth, are in phase across all three phases. Instead of cancelling in the neutral as balanced fundamental current does, they add. A neutral sized on the assumption that it carries only imbalance can end up carrying more current than any phase conductor, and it has no overcurrent protection of its own in many installations. This is the mechanism behind overheated neutral bars in densely lit rooms, and it is a harmonic problem rather than a load-balance problem.
IEC 61000-3-2 sets limits for harmonic current emissions from equipment drawing up to 16 A per phase. Lighting equipment falls into Class C, and the limits are expressed as a percentage of the fundamental current, scaled by the circuit power factor λ. The standard is the reason active correction is normal in professional luminaires: it is difficult to meet the Class C envelope with a passive front end at higher wattages.
Dimming moves both numbers
Power factor is quoted at rated load because that is where it is easiest to achieve. In a grow room, fixtures often spend most of the photoperiod at 40 to 70 percent output. At light load the correction stage works far from its design point, displacement factor falls and distortion rises, so the true power factor at 20 percent dimming can be 0.7 or lower on a driver that quotes 0.95.
Two practical consequences follow. First, size distribution equipment using the current at the dim level you actually run, not the headline figure. Second, when comparing quotations, insist that power factor and THD are quoted at 100, 50 and 20 percent output. A single number at full load is a marketing figure; three numbers are an engineering one.
What to ask before you sign
- Power factor and THD at 100, 50 and 20 percent output, at the supply voltage of your site.
- Whether the driver uses active or passive correction, and the driver manufacturer and model.
- The EMC test report, showing the standard applied and the class of limits.
- Recommended neutral and breaker sizing for arrays above roughly 20 kW.
- Behaviour on a generator, including inrush and whether a soft-start is fitted.
None of these questions is exotic. They are the same questions a factory would ask before installing motor drives, and a lit grow room is an electrical installation with a large nonlinear load before it is a garden.
About the publisher
XineLam has 17 years of experience in LED lighting and holds 300+ patents in China and internationally. The company designs and manufactures LED modules and horticultural luminaires for greenhouses, vertical farms and research rooms.
This article is a technical explainer. Power factor and THD are properties of a driver in a circuit; the figures quoted here are typical ranges, and the values for a specific product should be taken from its own test report.
Sources
Frequently asked questions
Does a higher power factor mean the fixture produces more light per watt?
No. Power factor describes the relationship between the current drawn and the power actually used; it says nothing about how efficiently electricity becomes photons. A driver with active correction can even be a fraction of a percent less efficient internally, because the correction circuit consumes a little power. The saving appears at the installation level: smaller cables, smaller generator and less heating in transformers and neutrals.
Should I install power factor correction capacitors in a grow room?
Usually not. Capacitor banks correct displacement power factor, but the current drawn by LED drivers is distorted rather than simply displaced, and capacitors can resonate with the harmonic currents that are already present. The robust answer is to specify drivers with active power factor correction and to size distribution equipment for the real current, harmonics included.
Is total harmonic distortion the same thing as flicker?
They come from the same driver but they are different problems. THD describes distortion of the current drawn from the mains, at multiples of 50 or 60 Hz. Flicker describes modulation of the light output, usually at twice the mains frequency or at the dimming frequency. A fixture can have excellent THD and still flicker, or the reverse.
What power factor should I expect from a horticultural LED driver?
Well-designed drivers with active correction typically reach 0.90 to 0.99 at full load on a 230 V supply. Below about 30 percent dimming the figure often falls, sometimes to 0.7 or lower, because the correction circuit is working far from its design point. Ask for the value at the dim level you intend to run, not only at 100 percent.