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

PAR, PPFD, DLI decoded. How to measure light at canopy level, which crops need how much, and how to size a light without overspending.

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 15-25, with basil at the upper end (around 18-25) and cilantro lower (12-16). Basil in particular gets leggy and loses flavor intensity below DLI 15.

Strawberries: DLI 17-22.

Peppers: DLI 22-30. Heat-loving plants that also demand strong light. Below DLI 18, flower drop increases and fruit set declines.

Tomatoes: DLI 22-30. 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.

What to look for when buying

Efficacy (micromoles per joule, umol/J). This is the LED equivalent of miles per gallon. It tells you how much usable plant light the fixture produces per watt of electricity consumed. Budget LEDs: 1.5-2.0 umol/J. Mid-range: 2.0-2.5 umol/J. Premium (Samsung LM301B/H diodes, MeanWell drivers): 2.5-3.0 umol/J. Higher efficacy means lower electricity cost for the same amount of light, and less heat to manage.

PPFD map at your intended height. A light that produces 500 PPFD at 45 cm may only produce 200 PPFD at 75 cm. The map tells you the coverage area at various heights so you can match the fixture to your grow space. A light that's perfect for a 60 x 60 cm tent may be inadequate for a 120 x 120 cm tent at any mounting height.

Spectrum. Modern full-spectrum white LEDs (3000-5000K color temperature) cover the entire photosynthetically active range. Supplemental red (660 nm) and far-red (730 nm) diodes enhance flowering and fruit set. Supplemental blue (450 nm) promotes compact growth. For vegetable production, a full-spectrum white with 10-20% supplemental red is a proven combination.

Dimmability. A dimmable driver lets you reduce light intensity for seedlings (which need 200-300 PPFD, not the 500+ that mature plants want) without raising the fixture. It also lets you fine-tune DLI without changing the photoperiod. A non-dimmable light is either on at full power or off.

Fixture types and form factors

Quantum boards (flat panel). A flat aluminum heatsink covered in densely packed SMD LEDs. The most efficient design currently available because the large surface area dissipates heat passively (no fans). Spider Farmer, HLG (Horticulture Lighting Group), and Mars Hydro all sell quantum-board-style lights. Sizes range from 50W (small tent or shelf) to 600W+ (commercial grow space). These have become the default for home growers because of their efficiency, even light distribution, and low profile.

Bar-style (multi-bar, spider). Multiple LED bars mounted on a frame, spread across the grow area. The bars provide more even coverage than a single point source. Common in commercial and large-tent applications. Gavita, Fluence, and Grower's Choice make commercial bar fixtures. Home versions from Mars Hydro, Spider Farmer, and others cost $200-500 for a 200-400W unit.

COB (chip on board). A single high-power LED chip produces intense light from a small area. Older technology, now largely superseded by quantum boards for most applications. COBs are still used for supplemental spot lighting and in fixtures where a concentrated beam is needed (e.g., illuminating a single large plant from above).

Fluorescent (T5, T8, CFL). Older technology. T5 high-output fluorescents were the standard before LEDs became affordable. They still work for seed starting and low-light crops (lettuce, herbs) but are less efficient (1.0-1.5 umol/J vs 2.0-3.0 for LED) and produce more heat per unit of useful light. For new purchases, LED is better in every measurable way. If you already have T5 fixtures, they're fine for seed starting and supplemental lighting.

Heat management

LEDs produce less heat per unit of light than HPS or fluorescent fixtures, but "less" is not "none." A 200W LED converts roughly 50-60% of its electrical input into light and the rest into heat. That's 80-100W of heat from a single fixture, enough to raise the temperature of a small enclosed grow space by several degrees.

Ambient temperature rise. In a 1.2 x 1.2 x 2 meter grow tent, a 200W LED raises air temperature by 5-10 C above ambient, depending on ventilation. An inline fan exhausting hot air from the top of the tent and drawing cool air in through a lower intake vent is standard. Size the fan for at least one full air exchange per minute of the tent volume.

Canopy temperature. The leaf surface directly under the light absorbs radiant energy and heats above ambient air temperature. Infrared thermometer readings of the canopy surface are typically 2-5 C above the air temperature at the same height. This matters for VPD calculations: use leaf temperature, not air temperature, for accurate VPD targeting.

Driver placement. Some LED fixtures allow the driver (power supply) to be mounted remotely, outside the grow space. Since the driver generates a significant portion of the fixture's total heat, placing it outside the tent reduces the cooling load inside. Not all fixtures support this; check before buying if heat management is a concern.

Summer vs winter. The heat that's a problem in summer is free heating in winter. Growers in cold climates sometimes welcome LED heat during winter months and only need supplemental cooling from late spring through early fall. Consider this seasonal dynamic when sizing ventilation; an exhaust fan on a thermostat-controlled speed controller adapts automatically.

How much to spend

The LED grow light market ranges from $30 no-name Amazon specials to $1500+ commercial fixtures. Price correlates loosely with efficiency, build quality, and lifespan, but there are diminishing returns above the mid-range.

Budget ($30-80, 50-150W). Basic quantum boards or small bar lights. Samsung LM281B+ diodes (not the top-tier LM301 but adequate). Generic drivers. Efficacy around 1.8-2.2 umol/J. Fine for lettuce, herbs, seed starting, and single-plant grows. These fixtures work, but they produce less light per watt and the drivers may have shorter lifespans.

Mid-range ($100-250, 100-300W). Samsung LM301B or LM301H diodes. MeanWell or equivalent brand-name drivers. Efficacy 2.3-2.8 umol/J. Dimmable. This is the sweet spot for most home growers. The fixtures last 50,000+ hours (5+ years of daily use) and the efficiency means lower electricity bills over their lifetime.

Premium ($300-600+, 300-600W). Top-tier diodes, commercial-grade drivers, UV and far-red supplemental channels, Bluetooth dimming controls, and extensive warranty. Efficacy 2.7-3.0 umol/J. Worth it for serious growers running commercial-scale production where the efficiency difference translates to meaningful electricity savings. For a hobby grower with a single tent, the mid-range provides 90% of the performance at half the price.

The math: A 200W mid-range LED ($150) running 14 hours per day at $0.15/kWh costs about $12.60/month in electricity. A 200W budget LED ($60) at 1.8 umol/J produces about 25% less usable light than a mid-range unit at 2.5 umol/J. To match the mid-range light's output, you'd need a 270W budget fixture, costing about $17.00/month. The mid-range light saves about $4.40/month in electricity, paying back its $90 price premium in roughly 20 months.

The grow light calculator translates fixture specs into DLI and coverage estimates for your grow area.