Grow lights are sold on watts, coverage claims and marketing words like "3000W equivalent". None of those tell you whether a light will grow plants well in your tent. Two numbers do, and once you know them the buying decision gets simple.
The two numbers that matter
PPFD — photosynthetic photon flux density — is how much usable light lands on the canopy each second, measured in µmol/m²/s. It's what the plant actually experiences. Manufacturers publish PPFD maps for their lights; look for one that covers your tent size at a realistic hanging height.
DLI — daily light integral — is PPFD added up over the hours the light is on. It's the "how much light did the plant get today" number, in mol/m²/day.
Rough targets, for most light-loving plants:
| Stage | PPFD (µmol/m²/s) | Hours | DLI (mol/m²/day) |
|---|---|---|---|
| Seedlings & cuttings | 100–300 | 18 | 6–19 |
| Vegetative growth | 300–600 | 18 | 19–39 |
| Flowering / fruiting | 600–900 | 12 | 26–39 |
| Leafy herbs & salad | 200–400 | 14–16 | 10–23 |
Two things to notice. First, flowering plants want roughly twice the light intensity of vegetating ones — which is why a light that seemed plenty in veg suddenly isn't. Second, herbs and leafy greens need far less; if that's your grow, don't buy a flowering-grade light.
Turning that into watts
PPFD maps are the right way, but a watts-per-square-foot rule gets you close for modern, full-spectrum LED (the white-diode bar or board style):
- Vegetative-only, herbs, greens: 15–25 W per sq ft
- Full grow through flowering: 30–40 W per sq ft
So for a full grow:
| Tent | Floor | LED wattage to aim for |
|---|---|---|
| 2×2 | 4 sq ft | 100–150 W |
| 2×4 | 8 sq ft | 200–300 W |
| 3×3 | 9 sq ft | 250–300 W |
| 4×4 | 16 sq ft | 400–500 W |
| 5×5 | 25 sq ft | 600–700 W |
That's actual wattage drawn from the wall — the number on the spec sheet as "power draw" or "actual power". Ignore anything described as "equivalent" wattage; it's a comparison to an old HPS lamp and tells you nothing about the LED.
Why "coverage" claims are usually optimistic
A light listed as "4×4 coverage" often means it will keep plants alive over 4×4 in veg. Its flowering coverage — the area it can hold at 600+ PPFD — is usually one size down. A light marketed for 4×4 flowering should be drawing 400 W or more; if it's 250 W, it's a 3×3 flowering light with an ambitious label.
The honest test is the PPFD map. If the manufacturer shows the corners of the claimed area at 400 when the centre reads 1000, you'll get uneven growth: tall in the middle, sluggish at the edges.
Hanging height and dimming
Most LED bars and boards run best 12–24 inches above the canopy. Closer gives more intensity but a smaller even patch; further gives better spread. A light with a dimmer lets you start seedlings at 30–40% and turn it up as the plant grows, rather than hanging it two feet away and losing light to the walls — dimming is worth paying for.
Watch for light stress as you push intensity: leaves cupping or tacoing upward, tips bleaching pale. Back off 10–20% and raise the light a few inches.
Spectrum, briefly
For general growing, a full-spectrum white LED (the kind with mostly white diodes and a few deep red ones) does everything you need. Purple "blurple" lights work but make plants hard to inspect and are mostly a sign of an older design. UV and far-red add-ons are refinements, not requirements — get the intensity right first.
What to actually buy
For a 2×2, a 100 W bar or board is the sweet spot: enough to flower one plant properly, dimmable for seedlings, and it won't heat the tent much.
For a 2×4, either one 200–250 W light or two 100 W lights. Two smaller lights give more even coverage across the rectangle and let you run one for seedlings.
For a 4×4, a single 400–480 W bar light. This is the size where diode quality and efficiency start to matter for your electricity bill — a modern high-efficiency light can draw 100 W less than an older one for the same output.
The one-line version
Multiply your tent's floor area by 30–40 W for a full grow, or 15–25 W for herbs and greens, buy a full-spectrum white LED that actually draws that much, and check its PPFD map covers your tent at 18 inches. Everything else is a refinement.
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