How to Choose LED Grow Lights for Cannabis: PPFD, DLI, Wattage

Quick answer

Judge a cannabis LED by three numbers: PPFD at the canopy, DLI for the growth stage, and efficacy in micromoles per joule (µmol/J). Wattage labels and "HPS replacement" claims tell you little. A flowering room needs 600 to 1,000 PPFD and 30 to 40 mol/m²/day.

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Light output facts to compare

• PPF: total photons per second, in µmol/s.
• Efficacy: PPF divided by wall watts, in µmol/J. Good fixtures run 2.5 to 3.0. The best current models reach 3.4.
• PPFD: photons per square meter per second at a set distance. Ask for the full PPFD map, not one center reading.
• DLI: PPFD × seconds of light ÷ 1,000,000. Example: 800 PPFD × 43,200 s (12 h) ÷ 1,000,000 = 34.6 mol/m²/day.

Match PPFD and DLI to the stage

• Clones and seedlings: 100 to 300 PPFD.
• Vegetative, 18 h photoperiod: 400 to 600 PPFD, which equals 26 to 39 mol/m²/day.
• Flowering, 12 h photoperiod: 600 to 1,000 PPFD, which equals 26 to 43 mol/m²/day.
• CO2 at 1,000 to 1,500 ppm: 1,000 to 1,500 PPFD. Returns fall off above about 45 mol/m²/day without CO2.

Measure PPFD at canopy height, not at the fixture. A quantum sensor (LI-COR, Apogee) reads PAR. Phone apps do not.

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Check the spectrum

PAR spans 400 to 700 nm. White diodes at 3000K to 5000K plus 660 nm deep red cover the main absorption peaks. Blue at 450 nm keeps vegetative growth compact. Far-red at 730 nm raises photon yield per watt and can shorten flowering time, but it also stretches stems. UV-B is not required and can cut yield.

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Size the fixture to the canopy

For a fixture at 2.5 µmol/J or better, plan 25 to 35 W per square foot of canopy. A 4 x 4 ft tent covers 16 sq ft, so flower needs about 400 to 560 W. Fixtures at 1.5 to 2.0 µmol/J need 40 W per square foot or more. Bar and strip designs spread PPFD across the canopy. Single-board lights drop off at the edges. Follow the maker's hang height chart.

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Heat, drivers, and certifications

At 2.5 to 3.0 µmol/J, about half of the input watts leave the fixture as heat. Budget for exhaust or AC. Look for Mean Well or Inventronics drivers, 0 to 10 V dimming, and IP65 or higher ratings in wet rooms. Safety marks: UL 8800 or ETL for horticultural luminaires. The DesignLights Consortium Horticultural QPL lists fixtures that meet minimum efficacy thresholds, and many utilities pay rebates only for listed models.

Cost math

A 480 W fixture at 12 h per day over a 60 day flower cycle uses 346 kWh (480 × 12 × 60 ÷ 1,000). At $0.15 per kWh that is about $52 per cycle. A fixture that costs $200 more but runs at 3.0 µmol/J instead of 2.0 delivers the same photons for a third less wattage.

Buying checklist

1. Efficacy at or above 2.5 µmol/J.
2. PPFD map that holds 600 to 1,000 across the canopy at your hang height.
3. Wattage sized at 25 to 35 W per sq ft.
4. Dimming plus a driver from a known brand.
5. UL 8800 or ETL listing, IP65 or better.
6. Warranty of 5 years or more.