Optimal Cannabis Grow Light Schedule: 18 Hours On, 6 Hours Off
Discover the best cannabis grow light schedule for optimal plant health and yield, with a focus on the 18/6 light cycle.
The short answer: for flowering cannabis, plan on 30 to 40 watts per square foot of canopy with a modern LED fixture, or 50 to 60 watts per square foot with HPS. A 4x4 tent holds 16 square feet of canopy, so it wants 480 to 640 watts of LED. That covers most home grows. What follows is when those numbers move, and why wattage by itself never tells the full story.
Optimal Cannabis Grow Light Schedule: 18 Hours On, 6 Hours Off
Wattage by grow space size
Canopy area drives the math, not plant count. Measure the footprint your light actually covers at the canopy, then multiply. These are LED figures for flowering:
- 2x2 ft (4 sq ft): 120 to 160W
- 2x4 ft (8 sq ft): 240 to 320W
- 3x3 ft (9 sq ft): 270 to 360W
- 4x4 ft (16 sq ft): 480 to 640W
- 5x5 ft (25 sq ft): 750 to 1,000W
- 4x8 ft (32 sq ft): 960 to 1,280W
Running HPS or CMH? Bump those numbers by 50 to 100 percent. A 1,000W double-ended HPS over a 4x4 is 62 watts per square foot, which is right in the sweet spot for that technology.
Watts per plant is a bad metric
I see growers ask "how many watts per plant" and the honest answer is that it depends on how big the plant gets. One scrogged plant filling a 4x4 wants the same 480 watts as four smaller plants in the same tent. The light covers area, and the plants divide that area up. Four plants in a 4x4 works out to 120 watts each. Sixteen plants in the same tent works out to 30 watts each. Same light, same yield potential.
If you must use a per-plant figure, treat 100 to 150 watts of LED per plant as a rough floor for medium plants in 5-gallon pots, and 200 to 300 watts for large, long-vegged plants.
LED vs HID vs "equivalent" watts
LEDs convert electricity to light more efficiently, so they need fewer watts to hit the same intensity. That efficiency gap is the whole reason the LED number is lower. HPS is still a fine flowering light. It just costs more to run and dumps a lot of heat into the tent.
Watch the marketing numbers on cheap panels. A fixture advertised as "1000W equivalent" often draws 110 watts at the wall. Look for the rated power draw on the spec sheet, and if it's vague, plug the light into a watt meter and read it yourself. That single habit will save you more money than any other buying decision.
Watts are not light: PPFD and DLI
Watts measure electricity going in. What the plant responds to is photons landing on leaves, expressed as PPFD (micromoles per square meter per second) and DLI (total light per day). A 480W bar light running at about 2.5 micromoles per joule puts roughly 800 PPFD on a 4x4 canopy. On a 12/12 flower schedule that is a DLI near 35 mol per square meter per day.
Targets for flowering without added CO2 fall around 600 to 900 PPFD, or a DLI of 30 to 45. Push past that in a sealed room with ambient CO2 and you get diminishing returns and light stress. A cheap lux meter or a phone app won't give you accurate PPFD, but a decent quantum sensor will, and it turns wattage guessing into measurement.
Vegetative vs flowering wattage
Veg does not need full flower intensity. Half the flower wattage is a good starting point: 15 to 25 watts per square foot of LED. Most people with a bar light just dim it to 50 or 60 percent and raise it a bit. Seedlings and clones want less again, closer to 100 to 200 PPFD, or you will cook them.
What moves the number up or down
- Fixture efficiency. A 2.7 micromole per joule light hits target intensity at 25 to 30 watts per square foot. A cheap blurple may need 45.
- Canopy shape. A flat, even scrog uses every photon. Tall untrained plants waste light on the walls.
- CO2 enrichment. Sealed rooms running 1,200 to 1,500 ppm can push 40 to 50 watts per square foot and higher PPFD.
- Mounting height and spread. Bars spread light better than a single point source. Going too high loses intensity at the edges.
- Veg time and strain. A 10-week veg fills the tent. A fast autoflower in a 3-gallon pot will not.
What it costs to run
A 480W light on 12/12 burns 5.76 kWh a day. At 17 cents a kWh that is about a dollar a day, or roughly $29 a month through flower. Veg at 240W and 18/6 runs about $0.73 a day. Add fans, exhaust, and a dehumidifier and you can tack on another 20 to 40 percent. Worth knowing before you buy a 1,000W HPS for a bedroom closet.
Quick checklist before you buy
- Measure the actual canopy footprint in square feet.
- Multiply by 30 to 40 for LED, or 50 to 60 for HID.
- Confirm the true wall draw, not the "equivalent" wattage.
- Check the efficiency rating in micromoles per joule if it's listed.
- Make sure the fixture is dimmable, so one light covers veg and flower.
Get those five things right and your lighting is settled. Everything else in the grow, from VPD to feed strength, gets easier to dial in once the light is where it should be. And if you would rather skip the tents and power bills entirely, buying from a licensed source is the shorter route.