Cannabis Grow Lights
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Most cannabis plants need 30 to 40 watts of true LED draw per square foot of canopy during flowering, and 20 to 25 watts per square foot during veg. In practical terms, a 4x4 tent (16 square feet) needs roughly 450 to 600 watts of LED, and a 3x3 tent (9 square feet) needs about 250 to 350 watts. Wattage is only a shortcut, because the plant responds to light intensity (PPFD) and total daily light (DLI), not to the number printed on the box.
How many watts per square foot do you need?
Use canopy area, not plant count, to set your target. A wide, well-trained canopy uses light far more efficiently than two tall plants in the same tent.
- Veg stage: 20 to 25 watts per square foot of true LED draw
- Flower stage: 30 to 40 watts per square foot of true LED draw
- High-yield or CO2-enriched rooms: 40 to 50 watts per square foot
- HPS or CMH: 50 to 60 watts per square foot, since these lamps convert less electricity into usable light
How many watts for common tent sizes?
These numbers assume quality LEDs pulling their rated wattage from the wall. Round up to the nearest fixture size rather than underpowering the space.
- 2x2 tent (4 sq ft): 100 to 150 watts
- 3x3 tent (9 sq ft): 250 to 350 watts
- 4x4 tent (16 sq ft): 450 to 600 watts
- 5x5 tent (25 sq ft): 700 to 1,000 watts
- 4x8 room (32 sq ft): 950 to 1,300 watts
Why advertised wattage can mislead you
Many budget fixtures market themselves as "1000W" or "2000W" while drawing only 100 to 200 watts from the wall. That label often describes an obsolete HID equivalent, not real power consumption. Check the actual power draw in the specs or plug the light into a watt meter before you size your setup.
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Efficiency matters just as much. A modern LED delivering 2.5 micromoles per joule produces roughly twice the light of an older lamp at the same wattage.
Do watts matter more than PPFD and DLI?
Watts measure electricity, while PPFD measures the light that actually reaches the canopy. A 400-watt LED with efficient diodes and good spread can outperform a 600-watt fixture with poor optics and hot spots. DLI then combines intensity and hours of light into a single daily target, which is why a slightly dimmer light run for 12 hours can beat a stronger one run for 8.
Rough PPFD targets for cannabis
- Seedlings and clones: 200 to 400 µmol/m²/s
- Vegetative growth: 400 to 600 µmol/m²/s
- Flowering without added CO2: 600 to 900 µmol/m²/s
- Flowering with CO2 enrichment: 900 to 1,200 µmol/m²/s
Can you run too many watts?
Yes. Above roughly 1,000 µmol/m²/s without CO2, extra light usually produces heat and light stress instead of more bud. Symptoms include bleached upper leaves, fox-tailing, tacoing, and drying tops. Dimming a large fixture is fine, but it wastes money you could have spent on better diodes.
How do you calculate wattage for your grow?
- Measure the canopy footprint in feet (length times width).
- Multiply square feet by 30 to 40 for flowering.
- Compare that number to the fixture's true wall draw, not its name.
- Confirm the circuit can handle the load, including fans, AC, and dehumidifiers.
A standard 15-amp, 120-volt circuit supplies 1,800 watts, and continuous loads should stay near 80 percent of that (about 1,440 watts). Two 600-watt LEDs plus climate gear can push a single bedroom circuit close to the limit.
How many watts for two plants?
Two plants in a 2x2 tent still need about 100 to 150 watts, because the tent walls cap the canopy. Two plants filling a 4x4 need 450 to 600 watts. Plant count only changes wattage when it changes the canopy area you are lighting.