Two LED grow lights are both labelled "1000W". One draws 100 watts from the wall and lights a 2×2 tent. The other draws 240 watts and lights a 2×4. Neither draws anything like 1000 W. If that seems like it should be illegal, it more or less is in the sense that nobody polices it — which is why understanding what the numbers mean is the difference between buying the right light and buying a name.
The three "wattages" you'll see
1. Marketing wattage — "1000W", "2000W", "600W HPS equivalent". Sometimes it's the theoretical maximum of every diode on the board (200 diodes × 5 W each = "1000W", never mind that they're driven at a fraction of that). Sometimes it's a claimed equivalence to an old HPS lamp. Either way it tells you nothing you can use. Ignore it completely.
2. Actual power draw — "power consumption", "true wattage", "draws 100W ±5%". The electricity it pulls from the wall. This is the number that determines your bill, the heat in the tent, and — combined with efficiency — how much light you get. This is the only wattage that matters, and a reputable maker prints it clearly. If a listing won't tell you the actual draw, that's your answer about the listing.
3. Diode wattage — "Samsung LM301B, 0.2W each". How hard each diode is driven. More diodes at lower current is more efficient and runs cooler than fewer diodes pushed hard. Useful for comparing two lights of the same actual draw, otherwise not something to shop by.
From watts to light: efficacy
A light doesn't grow plants with watts. It grows them with photons — specifically photosynthetically active ones, counted in micromoles per second (µmol/s, called PPF). How many photons you get per watt is the light's efficacy, in µmol/J:
Light output (PPF) = actual watts × efficacy
A 100 W light at 2.7 µmol/J puts out 270 µmol/s. A 100 W light at 2.0 puts out 200. Same electricity, 35% more light from the first. Efficacy is where the money in a light actually goes:
| Efficacy (µmol/J) | What it usually is |
|---|---|
| below 1.8 | Old "blurple" panels, cheap unbranded diodes |
| 2.0–2.4 | Older white LED designs, budget boards |
| 2.5–2.8 | Current mainstream (Samsung LM301B/H, Bridgelux EB) |
| 2.8–3.2 | Current top tier (LM301H EVO and similar) |
Reputable makers publish this number. If it's absent, assume the low end.
Output vs intensity: PPF and PPFD
PPF is the total light a fixture emits. What the plant experiences is PPFD — photons landing on each square metre of canopy each second. The same PPF spread over a 2×2 gives four times the PPFD it would over a 4×4. That's why "coverage" claims matter, and why a light's PPFD map — a grid of readings at a stated hanging height — is worth more than any wattage.
Reading a map: look at the corners of your tent size, not the centre. A light whose centre reads 900 and whose corners read 300 will grow a great plant in the middle and stragglers at the edges. Good uniformity is corners at 60% or more of the centre. And note the height the map was taken at — a map at 6 inches flatters every light.
Targets for the map, from How much light does a grow tent need: roughly 300–600 µmol/m²/s for vegetative growth, 600–900 for flowering.
The watts-per-square-foot rule
Once you're comparing actual draw on lights of current efficacy (2.5+), a simple rule gets you the right size:
- Vegetative, herbs, leafy greens: 15–25 W per square foot
- Flowering and fruiting: 30–40 W per square foot
| Tent | Floor | Full grow | Veg / greens only |
|---|---|---|---|
| 2×2 | 4 sq ft | 100–150 W | 60–100 W |
| 2×4 | 8 sq ft | 200–300 W | 120–200 W |
| 3×3 | 9 sq ft | 250–300 W | 140–220 W |
| 4×4 | 16 sq ft | 400–500 W | 250–400 W |
The rule breaks for old low-efficacy lights (they need more watts for the same result) — which is exactly why you check the efficacy first.
Dimming changes the sums
A dimmer scales the draw and the output together: a 250 W light at 50% draws about 125 W and gives about 125 W-worth of light. Two consequences:
- Buying bigger and dimming is fine. A 250 W light in a 2×2 at 50% is a 125 W light that can grow into a 2×4 later.
- Buying smaller and maxing it isn't. A 100 W light at 100% in a 2×4 is a 100 W light, and the tent's ends will be dim whatever the box says.
Dimming is also how you run one light from seedling (30–40%) to flower (100%) without moving it up and down.
Watts are heat
Almost every watt a light draws ends up as heat in the tent — either straight off the heatsink, or from the leaves after they absorb the light. So the actual draw is also your ventilation number: a 450 W light needs a fan sized for 450 W of heat, whatever the box calls it. Sizing a grow tent fan has the calculation.
Numbers to ignore
- Lumens and lux. They measure brightness as the human eye sees it, weighted to green, which plants use least. A light can be dim in lumens and excellent for plants, or vice versa.
- "Equivalent" or "replaces" wattage. Marketing.
- Diode count. Meaningless without knowing how hard they're driven.
- "Full spectrum" as a claim. Every white LED is; look at the spectrum chart if you care, and at the efficacy if you're choosing.
How to read a listing in thirty seconds
- Find the actual power draw. No number, no sale.
- Find the efficacy in µmol/J. Under 2.3, keep looking.
- Check the PPFD map at 18 inches: corners at 60%+ of centre over your tent size.
- Confirm it dims.
- Multiply your tent's floor by 30–40 and check the draw lands in that band.
If it passes all five, the brand on the front barely matters. Which LED for your tent compares the common size classes on exactly this basis.
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