Start with wall watts, not the number on the box
A light sold as "1000W" draws about 100W from the wall. The 1000W figure is a comparison to old HPS fixtures. Find the real power draw in the spec sheet or in a customer wattmeter photo. A budget LED with 100 real watts flowers a 2 ft by 2 ft area.
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Cost per wall watt sets your baseline. Budget boards run $0.60 to $1.00 per watt. A $75 fixture that draws 100W is a normal price. A $40 fixture that claims 1000W draws 40W or less.
Cannabis Grow Light Color Spectrum: What Blue, Red, and Far-Red Do
Measured light beats claimed light
PPFD is the number that matters. Ask for a PPFD map at 12, 18, and 24 inches. For flowering cannabis, aim for 600 to 900 umol/m2/s at the canopy top across the footprint. Check that corner readings hit 70% of the center reading.
cannabis grow light schedule for indoor
DLI combines intensity and hours. Cannabis in flower at 12/12 needs 30 to 40 mol/m2/day. Veg at 18/6 needs 20 to 30. A 100W board at 18 inches with a 12/12 schedule lands in that range.
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Parameters to compare
- Efficacy: 2.0 to 2.5 umol/J is normal for a cheap LED. Boards at 2.7 umol/J cost more.
- Driver: Mean Well or Inventronics drivers outlast unbranded drivers.
- Dimming: a knob or screw dimmer lets you run seedlings at 25% to 40% power.
- Spectrum: 3000K to 5000K white diodes plus 660 nm red diodes. Skip blurple panels.
- Thermal: passive heatsinks run silent. Fans add noise and one more part that fails.
- Coverage: 100W flowers 2x2 ft. 240W bar lights flower 3x3 ft.
- Warranty: 1 year minimum. 3 years is better.
Pitfalls
- Fake wattage. Cut the label number by 90% if no wattmeter photo exists.
- No PPFD map. If the seller lists no distance and no coverage data, skip that listing.
- No safety marks. Look for UL or ETL listing on the driver and the cord.
- Thin cords. A 16 AWG cord on a 240W light runs warm. Check the gauge.
- Wrong footprint. Two 100W boards beat one 200W board in a 4 ft tent, because the spread is wider.
Running cost and legal check
A 100W light at 12 hours uses 1.2 kWh per day. At $0.15 per kWh that is $0.18 per day, about $5.40 per month. A 240W light at 12/12 uses 2.88 kWh per day, about $13 per month.
Home grow rules differ by state. Some states cap plant counts and require a locked space. Check your state law before you buy hardware. A phone PAR app gives relative readings, so use it to compare spots in the tent, not as a lab number.