The Short Answer
Buy watts by canopy area, not by plant count and not by the number printed on the box. For modern LED fixtures, plan on 30 to 40 watts of actual wall draw per square foot of flowering canopy, and 20 to 30 watts per square foot for vegetative growth. If you still run a single-ended HPS, use 50 watts per square foot in flower and 30 to 40 in veg. A 4x4 tent, which is 16 square feet, needs about 480 to 640 LED watts at the wall to flower well. A 2x2, at 4 square feet, needs 120 to 160 watts. That math holds whether the fixture costs eighty dollars or eight hundred.
Watts are a budget proxy, not a light measurement. The fixture has to deliver enough photons to the canopy, spread them evenly, and do it without turning your tent into a space heater. Convenience has become a real deciding factor in this market, and it should be. App scheduling, built-in dimming, and daisy chaining save time every single day. Just sort on photon delivery first, then let convenience break the tie.
Why Wattage Claims Mislead
Many budget panels are marketed as "1000W" or "2000W" replacements while drawing 100 to 200 watts from the outlet. The label describes an outdated equivalence to blurple diodes, not the power the fixture consumes. Two fixtures with the same printed number can differ by a factor of three in real draw, and by a wider margin in usable photons.
The number that matters is photon efficacy, measured in micromoles per joule, usually written as µmol/J. Entry level LEDs land near 2.0, mid tier boards sit around 2.5 to 2.8, and premium bar fixtures reach 2.9 to 3.5. A 300 watt fixture at 3.0 µmol/J produces more usable light than a 400 watt fixture at 2.0. That is the whole argument for spending more per watt.
Cannabis Grow Light Schedule for Indoor: Veg, Flower, Auto
What to Look For Before You Buy
- Real draw at the wall. Look for input wattage in the specs, not the model name. A cheap plug-in power meter settles it in thirty seconds.
- PPFD map for your footprint. Ask for a chart measured at 12 to 18 inches above canopy across the full rectangle. Corner values within about 70 percent of center are workable.
- Dimming range. Zero to 100 percent control lets you run one fixture through veg and flower instead of buying twice.
- Driver placement. Remote or detachable drivers let you keep heat out of the tent, which matters in a closet or a small room.
- Thermal design. Passive bars with wide spacing run cooler and last longer than a single dense panel with one small fan.
- Service and warranty. Diode and driver failure is the common failure mode. A three to five year warranty with a US service address is worth real money.
Parameter Bands by Stage and Space
- Seedling and clone: 10 to 15 watts per square foot, or dim a larger fixture to 25 percent. Daily light integral target is roughly 10 to 15 mol/m²/day.
- Vegetative: 20 to 30 watts per square foot, PPFD of 400 to 600 µmol/m²/s, DLI of 20 to 30.
- Flowering, no CO2: 30 to 40 watts per square foot, PPFD of 700 to 1000 µmol/m²/s, DLI of 35 to 45.
- Flowering, sealed room with CO2: 40 to 50 watts per square foot, PPFD of 1000 to 1200, DLI up to 50 with temperature and humidity held in range.
- Common tent sizes: 2x2 needs 120 to 160 LED watts, 2x4 needs 240 to 320, 3x3 needs 270 to 360, 4x4 needs 480 to 640, 5x5 needs 750 to 1000.
Common Pitfalls
- Buying by plant count. Six small plants in a 2x2 still only fill 4 square feet of canopy.
- Ignoring hanging height. A fixture run at 8 inches can bleach tops while the corners stay dim.
- Skipping airflow. High PPFD without leaf movement and adequate vapor pressure deficit stalls growth.
- Chasing spectrum gimmicks. Full spectrum white with a modest red boost covers veg and flower. Deep blue or far-red add-ons rarely fix a weak fixture.
- Forgetting heat load. Every watt becomes heat. A 640 watt tent needs real exhaust, not a passive vent.
- Trusting a kit's bundled light. The tent and fan are usually fine. The included panel often is not.
FAQ
Is 100 watts enough for one cannabis plant?
Yes, if that plant occupies about 2.5 to 3 square feet and the fixture delivers 2.5 µmol/J or better. One large plant trained wide in a 3x3 will want 300 watts or more to fill the footprint evenly.
Do "1000W equivalent" panels actually work?
Some do at a small scale. A 100 watt draw over 2 square feet gives 50 watts per square foot, which is plenty. The problem is coverage. Most of these panels cannot spread light across a 4x4 without hotspotting the middle.
Can you give a plant too much light?
Yes. Without CO2 enrichment and tight climate control, pushing past roughly 1000 µmol/m²/s at the canopy often causes bleaching, leaf curl, and stalled growth. Dimming is the fix, not a taller tent.
Does wattage matter more than spectrum?
No. Intensity and uniformity drive yield more than narrow spectrum tuning. Get the wattage band right for your area, then pick a full spectrum fixture with a proven PPFD map.
Should convenience features decide the purchase?
They can decide between two fixtures that are already close on photons per dollar. Scheduling, dimming, and daisy chaining reduce daily labor and mistakes, which is a genuine return. They cannot rescue a fixture that under delivers on light.