What size grow light do I need?
One that puts enough photons on your canopy for the crop's daily light target in the hours the crop allows. That sentence has no watts in it, and neither should your shopping list until the last step: a fixture is sized by its PPF — photons out per second, µmol/s — and its efficacy tells you how many watts that costs. Two lights with the same PPF grow the same plants; the one with the higher µmol/J draws fewer watts, makes less heat and costs less every month it runs.
A 4 × 4 tent is 1.49 m². Flowering at the middle of the 35–50 mol band on a 12-hour day needs 984 µmol/m²/s; with 85% of the fixture's photons landing on the canopy in a reflective tent, that's a PPF of 1,720 µmol/s. At 2.3 µmol/J that is 748 W; at 1.5 it is 1,147 W. The target came from the DLI calculator's crop table; the 85% is an estimate, which is what the +20% headroom is for.
PPF and watts by tent size
| Tent | Greens, 16 h | Veg, 18 h | Flower, 12 h |
|---|---|---|---|
| 2 × 2 ft | 110 µmol/s 48 W · 12 W/sq ft | 219 µmol/s 95 W · 24 W/sq ft | 430 µmol/s 187 W · 47 W/sq ft |
| 2 × 4 ft | 220 µmol/s 96 W · 12 W/sq ft | 439 µmol/s 191 W · 24 W/sq ft | 860 µmol/s 374 W · 47 W/sq ft |
| 3 × 3 ft | 248 µmol/s 108 W · 12 W/sq ft | 493 µmol/s 215 W · 24 W/sq ft | 968 µmol/s 421 W · 47 W/sq ft |
| 4 × 4 ft | 440 µmol/s 191 W · 12 W/sq ft | 877 µmol/s 381 W · 24 W/sq ft | 1,720 µmol/s 748 W · 47 W/sq ft |
| 4 × 8 ft | 880 µmol/s 383 W · 12 W/sq ft | 1,754 µmol/s 763 W · 24 W/sq ft | 3,441 µmol/s 1,496 W · 47 W/sq ft |
| 5 × 5 ft | 688 µmol/s 299 W · 12 W/sq ft | 1,370 µmol/s 596 W · 24 W/sq ft | 2,688 µmol/s 1,169 W · 47 W/sq ft |
Middle of each band — greens 12–17 mol, veg 25–40, flower 35–50 — at 2.3 µmol/J (a quality LED board), 85% capture, no headroom. Add 20% to the PPF when shopping. A 4 × 8 is two 4 × 4 lights, not one big one — coverage, not photons, is the limit.
Why "watts per square foot" is the wrong unit
Every ranking page for this query says 30–50 W per square foot for flowering. That range is not a plant number; it's an efficacy spread. The photons a 4 × 4 flowering canopy needs are fixed — 1,720 µmol/s — and what that costs in watts depends entirely on the fixture:
| Fixture | µmol/J | Watts for 1,720 µmol/s | W / sq ft | Per month, 12 h at 15¢ |
|---|---|---|---|---|
| Fluorescent / CFL | 1 | 1,720 W | 108 | $92.90 |
| Budget LED (blurple, older boards) | 1.5 | 1,147 W | 72 | $61.94 |
| HPS, 1000 W double-ended | 1.7 | 1,012 W | 63 | $54.65 |
| Quality LED (Samsung LM301-class boards) | 2.3 | 748 W | 47 | $40.39 |
| Top-bin commercial bar light | 2.8 | 614 W | 38 | $33.18 |
4 × 4 ft, flower band middle, 12 h. The "30–50 W/sq ft" rule is just this table's middle rows. A budget panel needs 1.9× the watts of a top bar light for the same plants — and if it can't be bought that big, the plants get less light, which is the usual outcome.
For that 4 × 4 flower canopy, a 2.8 µmol/J bar light draws 614 W and a 1.5 µmol/J panel draws 1,147 W — the gap is $28.76 a month at 15¢ on a 12-hour day. If the good light costs $300 more, it has paid that back in 10 months, and it goes on saving for the years it outlasts the panel. It also dumps 1,817 fewer BTU an hour into the tent. Buy the µmol/J; the watts follow.
Reading the spec sheet
Four numbers, and an honest fixture prints all four: power draw in watts (from the wall, not "equivalent"), PPF in µmol/s, PPE in µmol/J (PPF ÷ watts — check the arithmetic; if it doesn't divide, one of the numbers is marketing), and a PPFD map at a stated hang height. Match the PPF to the calculator's number, then check the map's corners against the crop's PPFD — the corners are what the average hides. If a listing has a wattage in its name and nothing else, the decoder applies:
| What the listing says | Typical draw at the wall | What's going on |
|---|---|---|
| "1000W" or "2000W" marketplace panel | 100–250 W | The number is a claimed HPS "equivalent" — or nothing at all. Look for "power draw" in the fine print. |
| "600W" blurple panel | 60–120 W | Same game. Often ~1.2–1.5 µmol/J when measured. |
| "SF-1000"-style board (a model number) | 100–110 W | Named-brand boards state the true draw in the spec table; the model number is not a wattage. |
| Bar light listed by its real draw ("650 W") | 630–660 W | Honest fixtures list draw, PPF and PPE together. That is the tell. |
Typical figures from marketplace listings and teardown reviews, not a product database. The one durable rule: a fixture that lists draw, PPF and PPE together is telling the truth about all three, because you can check them against each other.
Seedlings need almost nothing
The same math in the other direction. A 2 × 2 propagation tray at the middle of the 4–11 mol band on an 18-hour day wants 116 µmol/m²/s — a PPF of 51 µmol/s, which is 51 W of ordinary fluorescent or 22 W of LED. A 100 W board over seedlings has to be dimmed or hung high, and a "1000 W" panel over a seed tray is how the leaves bleach in a week. Cheap shop lights are the right tool here; save the money for the flowering light.
How many fixtures?
One, up to about 4 × 4. Past that, photons aren't the problem — spreading them is. A 4 × 8 flowering canopy needs 3,441 µmol/s, which is exactly two of the 4 × 4 fixture, and two fixtures side by side light the middle of an 8-foot canopy in a way one bigger light hung higher never will. The calculator gives you the total; divide it by fixtures whose PPFD maps cover the footprint at your hang height, and let the corners of the map, not the total, decide.
Hang height and "coverage"
A manufacturer's "covers 4 × 4" is its veg footprint at 24 inches — raise the light and the footprint grows while the intensity falls (roughly with the square of the distance), lower it for flower and the same fixture covers a 3 × 3 at flowering PPFD. That is why this calculator sizes by photons over the whole canopy instead of by a coverage claim: the PPF you buy is the same at any height; what the height changes is how evenly it's spread. Multi-bar fixtures spread it more evenly than a single board, which is most of what you're paying for above ~2.3 µmol/J. Look for a PPFD map with corners within about 70% of the center at the height you'll use.
The heat you're also buying
Every watt of light becomes heat in the tent — the photons that hit leaves and walls end up as warmth too — at 3.412 BTU per hour per watt:
| Fixture draw | Heat | Roughly |
|---|---|---|
| 150 W | 512 BTU/h | a large laptop charger |
| 300 W | 1,024 BTU/h | a person at rest, twice over |
| 450 W | 1,535 BTU/h | a small space heater on low |
| 650 W | 2,218 BTU/h | a 5,000 BTU window AC running 40% of the time to remove |
| 1000 W | 3,412 BTU/h | a 5,000 BTU window AC running 70% of the time to remove |
This is the other reason efficacy matters: the 533 W a budget panel wastes on the same canopy is 1,817 BTU/h of extra exhaust or air conditioning.
When more light stops helping
The calculator will size a light for the top of any band you pick, but the honest ceiling in a tent breathing room air is about 50 mol a day — a flowering canopy past ~1,157 µmol/m²/s at 12 hours is being lit for CO₂ it doesn't have. Vegetables are lower still: a tomato's 18–30 mol is 417 µmol over a 16-hour day. If the number the calculator gives you looks small next to the lights people recommend, the lights people recommend are sized for CO₂ rooms — or for the arms race. Size for the crop, buy the efficacy, and spend the difference on airflow.
Sources: crop DLI targets as on the DLI calculator (Purdue HO-238-W, Cornell CEA, grower convention for cannabis, labeled); DesignLights Consortium horticultural technical requirements (PPE floor and reporting of PPF, input power and PPFD maps); manufacturer spec sheets for the efficacy classes; marketplace listings and teardown reviews for the advertised-watt decoder; 3.412 BTU/h per watt (unit conversion).