Short answer
Match the fixture to the tent footprint and to the stage of growth. For a 4x4 ft tent (16 sq ft), use 450 to 600 watts of LED at the wall. For a 2x2 ft tent (4 sq ft), use 100 to 150 watts of LED. HID lamps need more watts for the same area: a 600 watt HPS covers about 4x4 ft, a 400 watt HPS or a 315 watt CMH covers about 3x3 ft.
Full spectrum white LED runs veg and flower. T5 fluorescent runs seedlings and clones. HPS runs flower. Metal halide runs veg. A dimmer and a quantum sensor matter more than the brand name on the box.
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Light types
LED
Current LED bar fixtures run at 2.3 to 3.0 µmol/J. A 240 watt fixture at 2.7 µmol/J puts out about 648 µmol/s. Spread over 8 sq ft (2x4 ft), that is near 800 µmol/m²/s at the center before wall losses. Look for a UL or ETL mark, a dimmer that goes to 10 percent, and a driver you can mount outside the tent.
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HID: metal halide and high pressure sodium
HID fixtures cost less to buy and more to run. A 600 watt HPS with a digital ballast draws about 5.5 amps at 120 volts. A 1000 watt HPS draws about 9 amps. Lamps lose 10 to 20 percent of output by 10,000 hours, so plan on one bulb per year. HID needs an air cooled hood or a vented tent because the lamp radiates heat down onto the canopy.
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CMH
A 315 watt CMH covers 3x3 ft and draws about 2.6 amps at 120 volts. Output holds near 90 percent at 20,000 hours. Spectrum carries more blue than HPS, so one lamp can run veg and flower.
Fluorescent T5
A 4 bulb, 2 ft T5 HO fixture draws about 96 watts and covers 2x2 ft. Hang it 4 to 6 in above seedlings. T5 output is too low to flower a canopy above 2x2 ft.
Wattage by tent size, LED at the wall
- 2x2 ft, 4 sq ft: 100 to 150 W
- 2x4 ft, 8 sq ft: 200 to 300 W
- 3x3 ft, 9 sq ft: 250 to 350 W
- 4x4 ft, 16 sq ft: 450 to 600 W
- 4x8 ft, 32 sq ft: 900 to 1,200 W, two fixtures
These numbers assume 30 to 40 watts per sq ft and a fixture at 2.5 µmol/J or better. HID builds run 40 to 60 watts per sq ft: 600 W for 4x4 ft, 1000 W for 5x5 ft.
PPFD and DLI targets
PPFD is photon flux at the canopy in µmol/m²/s. DLI is the total for the day in mol/m²/day. DLI = PPFD x hours x 0.0036. At 600 PPFD on an 18/6 veg cycle, DLI is 38.9 mol/m²/day. At 800 PPFD on a 12/12 flower cycle, DLI is 34.6 mol/m²/day.
- Seedlings and clones: 100 to 300 PPFD, 18/6
- Veg: 400 to 600 PPFD, 18/6, DLI 25 to 35
- Flower: 600 to 1,000 PPFD, 12/12, DLI 30 to 45
CO2 above 800 ppm and a leaf zone at 75 to 80°F let a plant use the top of each range. Without added CO2, stay near the middle. Check PPFD at 9 points across the canopy with a quantum sensor. Phone apps drift by 10 to 20 percent, so calibrate against a meter.
Hanging distance
LED bar: 24 to 30 in in veg, 12 to 18 in in flower. 315 W CMH: 18 to 24 in. 600 W HPS in an air cooled hood: 18 to 24 in. 1000 W HPS: 24 to 36 in. Raise the fixture if leaf edges curl up, if leaf tips bleach, or if a hand at canopy height feels hot after 30 seconds.
Heat and electricity
Every watt that goes into the tent leaves as heat. A 600 W lamp adds 600 W of heat, about 2,050 BTU per hour. HID adds a second load because the hood dumps heat into the tent unless you vent it outside.
Run lights on a 15 amp circuit at 12 amps (1,440 W at 120 V). A 1000 W HID plus a fan, a pump, and a dehumidifier can pass that line. Use a GFCI outlet, a timer rated above the lamp load, and 14 AWG cord for runs under 50 ft. Keep ballasts and drivers off the floor.
Cost: at $0.15 per kWh, 600 W for 12 hours is 7.2 kWh, or $1.08 per day, about $32 per month.
Buying checklist
- Measure the tent footprint inside the poles.
- Pick 30 to 40 W per sq ft for LED, 40 to 60 W per sq ft for HID.
- Confirm a UL or ETL listing on the fixture and the driver.
- Check the dimmer range and the PPF or µmol/J number on the spec sheet.
- Add a quantum sensor, a timer, and a GFCI outlet.
- Plan a 12 in gap between the lamp and the tent ceiling for airflow.
State law sets plant counts and where you can grow. Check your state rules before you set up.