LED grow lights: what the specs mean and how much light your plants need

Grow light marketing is full of numbers that sound impressive and mean little. "1000W equivalent" is meaningless. "Full spectrum" is nearly every LED made after 2018. The number that matters is PPFD at canopy height, and the derived number from it, DLI, tells you whether your crop is getting enough light per day.

The units

PAR (photosynthetically active radiation). The wavelengths plants use: 400-700 nm. This is the relevant slice of the light spectrum for photosynthesis. A light that produces lots of lumens (visible brightness to the human eye) might produce relatively little PAR if the spectrum is weighted toward green, which plants reflect rather than absorb.

PPFD (photosynthetic photon flux density). The number of PAR photons hitting a surface per second, per square meter. Measured in umol/m2/s (micromoles per square meter per second). This is the intensity measurement you want from a grow light spec sheet. A reading of 200 umol/m2/s at 30 cm hanging height means 200 micromoles of photosynthetically useful photons hit each square meter of canopy per second at that distance.

DLI (daily light integral). Total PAR photons received per square meter per day. Measured in mol/m2/day. This is PPFD multiplied by the number of seconds the light runs, divided by a million.

Formula: DLI = PPFD x photoperiod_hours x 3600 / 1,000,000

Example: 200 umol/m2/s for 16 hours = 200 x 16 x 3600 / 1,000,000 = 11.5 mol/m2/day.

DLI is the number the plant actually cares about. A crop that needs DLI 17 can get it from moderate intensity for a long photoperiod (150 PPFD for 18 hours) or high intensity for a short one (300 PPFD for 10 hours). The total matters more than the instantaneous rate, with one caveat: PPFD above the plant's light saturation point wastes electricity. Lettuce saturates around 300-400 umol/m2/s; photons above that don't increase growth.

DLI targets by crop

These numbers come from controlled-environment agriculture research (University of Arizona CEAC, Cornell CEA, and Wageningen University). The grow light calculator and lighting calculator use the same data.

Lettuce and leafy greens: DLI 12-17. Tolerant of lower light; growth slows below DLI 10 but the plant doesn't fail. The most forgiving crop for a too-small light.

Herbs (basil, cilantro, parsley): DLI 15-20. Basil in particular gets leggy and loses flavor intensity below DLI 12.

Strawberries: DLI 17-22.

Peppers: DLI 20-30. Heat-loving plants that also demand strong light. Below DLI 18, flower drop increases and fruit set declines.

Tomatoes: DLI 22-35. The most light-hungry common hydroponic crop. Below DLI 20, tomato plants grow vegetatively but won't set fruit reliably. A single tomato plant under lights needs 150-250W of LED over its canopy.

Measuring light

A dedicated PAR meter (Apogee MQ-500, several hundred dollars) is the gold standard but overkill for most home growers. Phone apps (Photone, Korona) use the phone camera sensor to approximate PPFD. They're accurate to within 10-20% for white LEDs when calibrated, which is close enough for crop planning. The free tier of Photone is adequate; pay for the diffuser attachment if you want better accuracy.

Measure at canopy height, not at the light fixture. PPFD drops with the square of distance: doubling the distance between light and canopy quarters the intensity. A light rated at 800 PPFD at 15 cm might only deliver 200 PPFD at 30 cm. Always check the manufacturer's PPFD map at your intended hanging height.

Sizing a light

Work backward from the DLI target and your planned photoperiod:

1. Pick the crop with the highest DLI demand in the grow area.
2. Choose a photoperiod (14-18 hours for most crops; 12 hours for short-day plants like cannabis or some strawberry varieties).
3. Calculate the minimum average PPFD: required PPFD = DLI x 1,000,000 / (photoperiod_hours x 3600).
4. Find a light that delivers that PPFD or higher across the grow area at the planned hanging height.

Example: Tomatoes need DLI 25. At 16 hours: 25 x 1,000,000 / (16 x 3600) = 434 umol/m2/s average across the canopy. That's a serious light. For a 60 x 60 cm grow area, a 150-200W LED panel is the right size.

For lettuce at DLI 14 and 16 hours: 14 x 1,000,000 / (16 x 3600) = 243 umol/m2/s. A 100W panel covers a 60 x 60 cm lettuce area with headroom.

Spectrum

Modern white LEDs (Samsung LM301 diodes, Osram, Cree) produce a broad white spectrum with peaks around 450 nm (blue) and 620 nm (red) that covers the full PAR range. This is adequate for all crop types.

Older "blurple" lights (separate red and blue LEDs, no white) produce a purple-pink glow that hits the photosynthetic absorption peaks but misses the green wavelengths that contribute to deeper canopy penetration and photomorphogenesis. Blurple lights work but are less efficient per watt than modern white LEDs and make it harder to visually inspect plant health (everything looks purple).

Far-red (700-750 nm) supplementation is a current trend in commercial horticulture. Far-red extends the usable spectrum and triggers shade-avoidance responses that can increase stem elongation and flowering. For home growers, it's a marginal optimization; a good white LED is enough.

Efficiency

LED efficiency is measured in umol/J (micromoles of PAR photons per joule of electricity). Modern top-tier LEDs hit 2.5-3.0 umol/J. Budget lights run 1.5-2.0 umol/J. The difference adds up in electricity cost over a crop cycle.

A 150W light at 2.5 umol/J produces 375 umol/s. At 16 hours per day, that's 0.15 kW x 16 h = 2.4 kWh per day. The monthly electricity cost is modest for a single light. A less efficient light drawing 200W to produce the same PPFD costs noticeably more to run, and the difference compounds over a year with multiple lights. For a multi-light grow room, the math scales and efficiency pays for the higher upfront cost of premium diodes.

The grow light calculator estimates both PPFD and running cost for your specific light, hanging height, and photoperiod.