LED Grow Light Wattage by Tent Size (and LED vs HPS)

How many watts of LED grow light does a tent need? The honest answer is a range, because it depends on what you grow and how hard you push it, but the range is well established and it makes choosing a light straightforward. This guide gives the rule of thumb, a table by tent size, the fixtures we stock that fit each one, and a plain explanation of LED versus HPS and of PAR and PPFD.

The rule of thumb: watts per square metre

For a modern LED fixture of decent efficiency (about 2.5 to 3 µmol/J, which all the lights below are), plan on 300 to 500 W per square metre of canopy for flowering and fruiting. The bottom of the range grows a good crop; the top is for growers chasing the maximum from the space, and it needs the extraction, and ideally the CO₂, to go with it. For vegetative growth, 150 to 250 W/m² is plenty, and seedlings and cuttings want far less again: a 20 W Cosmorrow strip or a T5 covers a propagator.

Two things make the rule work. First, buy a dimmable fixture and run it at 25 to 50 per cent in veg, full in flower: one light, whole cycle. Second, match the fixture's footprint to the tent, not just its watts. A 720 W light hung in a 60 cm tent cannot be turned down far enough and wastes most of what it makes on the walls.

LED wattage by tent size

Floor areas are the tents we sell. The lights named are the ones on our shelves whose makers' own coverage figures fit that size.

The lights, with the makers' numbers

• LUMii Switch Blade 150 W: a slim full-spectrum bar for small to medium spaces; several can be combined for a bigger area.
• Maxibright Daylight 200 W PRO: 500 µmol/s at 2.5 µmol/J, covers 0.6 × 0.6 to 0.8 × 0.8 m, dims to 25 per cent.
• Maxibright Daylight 300 W PRO: 750 µmol/s at 2.5 µmol/J, covers 0.8 × 0.8 to 1 × 1 m, dims to 25 per cent. The Daylight 300 W is the fan-less panel version.
• Maxibright Varidrive 360 W: 1040 µmol/s at 2.89 µmol/J, covers 0.8 × 0.8 to 1.2 × 1.2 m; Maxibright put it on a par with a 600 W HPS. This is the light in our 1 m grow tent kit.
• Maxibright Daylight 480 W PRO: 1296 µmol/s at 2.7 µmol/J, covers 1 × 1 to 1.2 × 1.2 m. The nearest thing to a "400 W" LED and the right size for a 1 m tent pushed hard.
• Maxibright Daylight 660 W PRO: 2.7 µmol/J, covers 1.2 × 1.2 to 1.5 × 1.5 m, iLink for the Daylight controller.
• Maxibright Varidrive 3.0 720 W: 2160 µmol/s at 3.0 µmol/J, the most efficient fixture here, covers 1.2 × 1.2 to 1.5 × 1.5 m, folds for the box.
• Adjusta-Watt Intec+ 720 W: 1944 µmol/s at 2.7 µmol/J, 110 × 102 cm, made for a 1.2 × 1.2 m tent.
• Gorilla 720 W: 1944 µmol/s at 2.7 µmol/J, six bars, covers up to 1.5 × 1.5 m.
• Maxibright Daylight 1030 W PRO: the 1000 W class fixture for a 1.5 m tent or as one of several in a room; see the product page for its output.

Several fixtures run together from a lighting controller (Maxibright Daylight or Varidrive) with one schedule and sunrise and sunset dimming.

LED versus HPS

A 600 W HPS such as the Sunmaster 600 W Super HPS (95,000 lumens) flowered a 1.2 m tent for twenty years and still does. Against it, an LED of 480 to 720 W gives the same or more usable light for the same or less electricity, runs far cooler, so the extraction can be smaller and the light can hang closer, and needs no lamp change (HID lamps fade and want replacing every eight to twelve months). The LED costs more to buy; the electricity and lamps pay it back over a couple of years. HID still costs least up front, and a Varidrive ballast (250, 400, 600 W and a 660 W boost) with a Euro reflector is a cheap, proven way to flower a tent. Many experienced growers now run both: read why the best growers use LED and HPS together.

PAR and PPFD, in plain English

Watts measure what a light draws from the wall, not what the plant gets. PAR (photosynthetically active radiation) is the light plants can use, roughly the visible spectrum from deep blue to far red. PPF, in µmol/s, is how much of that a fixture puts out in total; efficiency in µmol/J is PPF divided by watts, which is how a 720 W fixture at 3.0 µmol/J (2160 µmol/s) beats one at 2.7 (1944 µmol/s) on the same electricity.

PPFD, in µmol/m²/s, is what lands on a square metre of canopy: the number that matters to the plant, and it changes with hanging height. As a guide, seedlings and cuttings want 100 to 300, vegetative growth 300 to 600, and flowering 600 to 900 (more with added CO₂). A PAR meter reads PPFD directly, so you can raise or dim the light to the number rather than guessing; it works with LED, HPS and CMH alike.

Quick answers

Is a 1000 W LED grow light too much for a 1.2 m tent?

Yes, unless it is dimmed. 1000 W over 1.44 m² is nearly 700 W/m²; a 660 or 720 W fixture is the right size, and a 1030 W fixture suits a 1.5 m tent or a room.

What does "full spectrum" mean?

White LEDs plus extra red and far red, so one light covers veg and flower rather than a blue lamp for one and a red lamp for the other. All the fixtures above are full spectrum.

How far above the plants should an LED hang?

Follow the maker's card, usually 30 to 45 cm from the canopy at full power, higher over seedlings or when dimmed. A PAR meter settles it.

Browse all grow lights, LED grow lights and HPS and metal halide.

More set-up guides

• What size grow tent do I need?
• Sizing an extraction fan and carbon filter
• LED or HPS? Why the best growers use both
• Hydroponics: systems, kits and how it works

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.