Grow Light FAQ

It depends on the plant, and the published bands are narrower than the guesswork suggests. University of Minnesota Extension puts low-light plants at 50 to 150 µmol/m²/s, medium at 150 to 250, and high at 250 to 450. Seedlings want a daily total of about 12 moles per square meter per day, per University of New Hampshire Extension. The full mapping is in PPFD targets by plant type.

Multiply your target light level by your area in square meters and divide by 0.8. A 2 × 3 ft shelf at a medium level needs about 139 µmol/s; at a high level, about 244. Both are inside the range of a 100 W panel. The table of every common area is in what size grow light do I need.

Wattage is an unreliable proxy now that efficacy varies by nearly a factor of two between fixtures — across the lights we cover it runs from 1.39 to 2.75 µmol/J. The same 30 W-per-square-foot rule gives an answer that is off by a factor of two depending on which fixture you buy. Size on photons instead.

Practical starting points: 2 to 4 inches for seedlings under low-output bars, 12 to 18 inches for seedlings under a concentrated panel, 8 to 14 inches for herbs and greens, and 12 to 24 inches for foliage houseplants. Intensity falls with roughly the square of distance, so halving the height roughly quadruples the light. Details and the published data are in how far a grow light should be from plants.

Between about 10 and 16 for almost everything a home grower keeps. University of Minnesota Extension recommends 16 to 18 hours for seedlings; Iowa State University Extension suggests 12 to 14 generally, with 10 as a floor and 16 as a ceiling. Never 24 — plants carry out processes in darkness that do not happen in light.

Nothing enforceable. There is no regulatory definition, no minimum spectral coverage and no certifying body, so every fixture in the category can truthfully print it — which makes it useless for choosing between two of them. Read PPF, efficacy and PPFD-with-a-distance instead. The argument is in what full spectrum actually means.

Not for growth, on the evidence we rely on: a peer-reviewed comparison in PLOS ONE by Zhen, van Iersel and colleagues found white LEDs performed comparably with blue-plus-red radiation while scoring far better on visual color quality. White wins at home for a practical reason — you can see chlorosis, mites and overwatering under it.

PPF is total photons leaving the fixture per second (µmol/s). PPFD is photons landing on a square meter of your plants (µmol/m²/s) — PPF divided by area. DLI is PPFD accumulated over a day (mol/m²/day). The formulas are in PAR, PPFD and DLI explained.

Not reliably. Lumens weight light by human eye sensitivity, which peaks in green where plants respond least, so the ratio to photon flux depends entirely on the spectrum of the specific fixture. Any single conversion factor is an approximation with unstated error — which is why a shop light specified only in lumens cannot be sized arithmetically.

Less than nearly everyone expects. A 100 W fixture at 14 hours a day uses about 42 kWh a month, roughly $7.70 at the US residential average of 18.34 cents per kWh. A whole three-shelf rack across an October-to-March winter comes to about $48. Price your own in the running-cost calculator.

The headline is usually the sum of the nominal ratings of every diode — run well below capacity, which is good engineering — or a claimed equivalence to an older HID lamp. Neither is what your meter records. Find the specification table, which for the better brands reads like “100 W ±5% at 100–277 V AC.”

A bright white household bulb within about a foot of one small plant is genuinely delivering usable light. Over a shelf, or at ceiling-lamp distances, it is not. The deeper problem is that household bulbs publish lumens, so you cannot size one by arithmetic — the difference from a grow bulb is as much about the data as the light.

On the published specifications of the nine fixtures we cover, no. Their stated spectral ranges begin at or above 400 nm, which is the bottom of visible light, and not one advertises a UV diode. One publishes a range starting at 380 nm without characterizing that portion further. Details per fixture are in do grow lights emit UV.

Photobiological safety is formally handled by IEC/EN 62471, which classifies light sources by risk group based on exposure duration and distance rather than brightness alone. The practical instruction is not to look into the emitters: mount the fixture above head height and put it on a timer. For pets we found no published species-specific ocular guidance and will not extrapolate one — the physical setup, cables and water are the hazards worth managing.

Warm, yes. Essentially all the electricity a fixture uses becomes heat, so a 100 W LED is a 100 W heat source. What differs from HID lighting is that most of it goes backwards into the heat sink rather than radiating down onto your plants — which is why heat scorch is uncommon and faster drying of the growing medium is the effect you actually notice.

Measure the area. If its longest side is more than about twice its shortest — a shelf, a seed tray, a windowsill — buy bars, because their emitters run along the shape you are lighting. If it is roughly square, buy a panel, which is also where the published photon data lives. Full argument in bar versus panel coverage.

No. We have not owned, unboxed or measured any fixture on this site, and we say so on every page rather than in a footer. Every figure is quoted from a published manufacturer specification with a link, or computed by us from those figures with the arithmetic shown. The full procedure is on the methodology page.

Because form factor decides more outcomes than specification does. A 4 ft strip over a 4 ft shelf distributes light along the shape you are lighting, which a square panel cannot, and that argument survives the missing data. We recommend it while stating in the same paragraph that its output cannot be verified before purchase — three of the nine fixtures we cover publish no photon figure at all.

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