What wattage LED do I need for my tent?

Q: What wattage LED do I need for my tent?

For flower, plan on roughly 25–40 W of efficient LED (2.5–3.0 µmol/J) per square foot, about 270–430 W/m², which gives an average of about 600–900 µmol/m²/s. That's 100–150 W for a 2 × 2 ft tent, 225–320 W for 3 × 3, 200–300 W for 2 × 4 and 400–650 W for 4 × 4. Experienced growers pushing 1,000–1,200 µmol/m²/s need about 35–50 W/ft².

Short answer

For flower, plan on roughly 25–40 W of efficient LED per square foot (about 270–430 W/m²) at 2.5–3.0 µmol/J, which delivers an average of about 600–900 µmol/m²/s. That's 100–150 W for a 2 × 2 ft tent, 225–320 W for 3 × 3 and 400–650 W for 4 × 4. Supported for the light–yield link; the watt rules are Grower practice.

Why

Yield indoors tracks light. In controlled trials, dry flower yield rose linearly from 120 to 1,800 µmol/m²/s at ambient CO₂ ^[1]^[2]. More light buys more grams, up to what the plant, the climate and your power bill can handle.

Watts are only half the picture. A fixture's output is watts × efficacy (µmol/J). Good current LEDs exceed 2.5–3 µmol/J, HPS about 1.7 ^[3]. The same wattage can deliver very different light.

Rough check: average PPFD ≈ watts × µmol/J ÷ canopy area, a little less in practice, and less in the corners.

The numbers

Tent	Area	LED for 600–900 µmol/m²/s	LED for 1,000–1,200 (CO₂ or experience)
2 × 2 ft (60 × 60 cm)	0.36 m² (4 ft²)	100–150 W	150–200 W
3 × 3 ft (90 × 90 cm)	0.81 m² (9 ft²)	225–320 W	320–450 W
2 × 4 ft (60 × 120 cm)	0.72 m² (8 ft²)	200–300 W	300–400 W
4 × 4 ft (120 × 120 cm)	1.44 m² (16 ft²)	400–650 W	550–800 W
5 × 5 ft (150 × 150 cm)	2.25 m² (25 ft²)	650–1,000 W	900–1,250 W

Figures assume 2.5–3.0 µmol/J. At 2.0 µmol/J, add about a third.

What to do

Measure the footprint you'll actually fill with canopy.
Pick a fixture whose published PPFD map covers that footprint at 600–900 µmol/m²/s average.
Check its µmol/J; compute watts × µmol/J ÷ area as a sanity check.
Buy a dimmable light and set it by measured canopy PPFD.
Check the circuit: lights plus everything else under 80 % of the rating.

Common mistakes

Buying by "equivalent watts".
Pushing above about 1,200 µmol/m²/s without CO₂ and healthy roots.
Ignoring corners: edges can get half the centre's light.

Full article: PPFD and DLI by stage
What should I look for when buying a grow light?
What size grow tent should I get?
Guide: Lights for beginners
Terms: photon efficacy · PPFD

Follow-up questions

How many watts do I need for a 4x4 tent?

A 4 × 4 ft (120 × 120 cm) tent needs about 400–650 W of efficient LED at 2.5–3.0 µmol/J for flower, which gives an average of roughly 600–900 µmol/m²/s across the canopy. Experienced growers aiming for 1,000–1,200 µmol/m²/s need about 550–800 W. Judge the fixture by its measured PPFD map over a 4 × 4 footprint, not by 'equivalent' watts.

Can I use a bigger light dimmed down?

Yes, a bigger LED dimmed down works well in a smaller tent and often runs more efficiently than a small light at full power. Set the dimmer by measured PPFD at the canopy, not by percentage. The catch is footprint: a fixture designed for 4 × 4 ft hung in a 2 × 2 tent wastes light on the walls and may be too hot for the space.

Do watts matter or only µmol/J?

Watts tell you what a grow light costs to run; µmol/J tells you how much plant-usable light each watt makes. Multiply them to get the light's output: a 300 W fixture at 2.7 µmol/J makes about 810 µmol/s of photons. Two 300 W lights at 1.7 and 2.8 µmol/J differ by about 65 % in light delivered. Always compare both numbers.

Sources

Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. doi.org/10.3389/fpls.2021.646020

Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y, 2022. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science 13:974018. doi.org/10.3389/fpls.2022.974018

Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. doi.org/10.1038/s41438-020-0283-7