Most LED grow lights come as a family — a 100 W model, a 200 W model, a 450 W model — and the question people actually type into a search box is "which one do I need?" The answer depends on your tent and what you're growing far more than on the brand. This is the comparison by size, with the numbers that decide it.
The short answer
| Your tent | Full grow through flowering | Veg, herbs, greens only |
|---|---|---|
| 2×2 | 100–150 W class | 60–100 W class |
| 2×4 | 200–260 W class, or 2 × 100 W | 150 W class |
| 3×3 | 250–300 W class | 150 W class |
| 4×4 | 400–480 W class | 250 W class |
"Class" means actual power drawn from the wall. Every reputable maker has a light in each of these bands, and within a band they perform more alike than the marketing admits.
The 100 W class: the 2×2 light
This is the most popular grow light size for a reason: it flowers one full plant in a 2×2, vegetates a 3×3, runs cool, and costs about the same as a decent kitchen appliance. Typical spec: 90–110 W draw, Samsung or Bridgelux white diodes with a few 660 nm reds, a dimmer, a passive heatsink with no fan.
Where it falls short: put it in a 2×4 and the ends of the tent get about half the light of the middle. Plants at the edges stretch and flower thin. It's not a 2×4 light, whatever the box says about "coverage".
The 200–260 W class: the 2×4 light
Same diodes, roughly double the board area. Made for a 2×4 rectangle, and this is where shape matters: a light that's itself rectangular (two bars, or a long board) spreads across a 2×4 far better than a square board hung in the middle. If you're buying one light for a 2×4, buy a rectangular one.
The alternative is two 100 W lights side by side. It costs a little more, but each half can be dimmed separately — seedlings under one, a flowering plant under the other — and if one fails you're not dark. For a first 2×4 it's the more flexible choice.
The 400–480 W class: the 4×4 light
This is the size where the design changes. Almost every good 4×4 light is now a bar light — six or eight thin LED bars on a frame — because bars spread light evenly to the tent corners and shed heat without a fan. A single square board this powerful would scorch the middle and starve the edges.
At this wattage, efficiency starts to show up on your electricity bill. Look for the efficacy figure, in µmol/J: 2.5 and above is current-generation; 2.0 or below is older tech drawing perhaps 100 W more for the same light. Over a year of 12-hour days that's real money.
Reading a spec sheet without being fooled
- "Actual power" or "power draw" is the number. Ignore "equivalent" or "replaces 600 W HPS".
- PPFD map at your hanging height should show the corners of your tent at 60% or more of the centre reading. If the map is only shown at 6 inches, the light is being flattered.
- Diodes: Samsung LM301B/LM301H and Bridgelux EB are the well-regarded names. Unnamed diodes aren't necessarily bad, but you're taking it on trust.
- Dimmer: essential. You'll run at 40% for seedlings and 100% only in late flower.
- Passive cooling: a light with no fan has nothing to fail and nothing to hear. Above 500 W some fans are unavoidable.
- Daisy-chaining: lets several lights run off one dimmer. Handy in a 4×4 with two units; irrelevant in a 2×2.
Bar vs board
| Board (quantum board) | Bar light | |
|---|---|---|
| Best at | 2×2, 3×3 | 2×4, 4×4 and up |
| Spread | Bright centre, dimmer edges | Even across the rectangle |
| Heat | Concentrated on one plate | Spread across bars |
| Price | Lower per watt | Higher, but more uniform |
| Fits under a low ceiling | Yes, slim | Slightly deeper frame |
For a square 2×2 or 3×3, a board is fine and cheaper. For any rectangle or anything 4×4 and up, bars win on evenness — and uneven light is the most common reason one plant in a tent does worse than the rest.
What I'd buy
- 2×2: a 100 W dimmable board or small bar. Don't over-buy; a 200 W light in a 2×2 will run at half power forever.
- 2×4: two 100 W lights, or one 250 W bar. Not one 100 W in the middle.
- 4×4: a 450 W-class bar light with a published efficacy of 2.5 µmol/J or better. This is the one purchase where paying for the newer design pays you back in electricity.
Match the light to the tent floor, check the PPFD map, and buy the dimmer. Brand loyalty matters less than any of those three.
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