What Size LED Do You Actually Need for Your Tent?

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Grow lights are sold by the number on the box: 100 watts, 1,000 watts, "equivalent" to this or that. But your plants don't read the box. They count the light that lands on them. So how much light should you aim for, how do you turn that into real watts, and why can the number on the box fool you?

The short answer ▶ 0:29

Pick a light level to aim for. Multiply it by your floor area. Divide by how efficient the light is. That gives you the real watts you need. Then round up a little.

How much light to aim for ▶ 0:40

As we covered in how much light seedlings need, growers measure light as PPFD: the photons landing on each square metre every second. Seedlings need about 150 to 200. A flowering plant can use far more.

In a 2021 trial in Canada, cannabis plants were flowered under LEDs at light levels from 120 all the way up to 1,800.

Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021)

The harvest rose in a straight line, the whole way up. At the top level, the plants gave 4.5 times as much dried flower per square metre as at the bottom.

On the study's chart, every extra 100 units of light added about 24 grams of flower per square metre. And that was in normal room air, with no extra carbon dioxide.

Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021), Fig. 7A

What about strength? Brighter light didn't change the THC or CBD content. It just gave more flower. The terpenes did rise, by about a quarter.

Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021)

So more light means more harvest, but every step up costs electricity and adds heat. That's why there's no single right number. In the sum below we aim for 800, a level many home growers can run. If you can pay for more power and get the heat out, more light still pays.

The sum: real watts for your tent ▶ 1:56

Start with the light you need. 800 over 1 square metre of floor means the light has to put out 800 micromoles every second. Twice the floor, twice the light.

Next: how much light does a lamp make from each watt? That's its efficiency, in micromoles per joule (µmol/J). A joule is one watt for one second, so a light rated 2.5 µmol/J turns every watt into 2.5 micromoles, every second.

Source · Kusuma, Pattison & Bugbee, Hortic. Res. (2020)

In a 2020 review, a big double-ended sodium lamp made about 1.7. To be listed by the DesignLights Consortium today, a horticultural light needs at least 2.3. And the best white LED fixtures made 2.5 to 2.8.

Source · Kusuma, Pattison & Bugbee (2020); DLC Horticultural V3.0

Now divide: 800 ÷ 2.5 = 320. So 1 square metre needs about 320 real watts from a good LED.

For common tent sizes, that's:

• 60 × 60 cm: about 115 W
• 80 × 80 cm: about 200 W
• 100 × 100 cm: 320 W
• 120 × 120 cm (about 4 × 4 ft): around 460 W

Then round up a little. Some light misses the plants and lands on the walls and floor, and the edges of a tent always get less than the middle. A dimmable light that's slightly too big is easy to turn down.

Growers often quote a rule of thumb: about 30 real watts per square foot. With a 2.5 µmol/J light that works out at around 800, so the rule and the sum agree. With a less efficient light, they don't.

Efficiency matters as much as size. To give the same square metre the same 800, a light rated 1.7 needs about 470 watts. That's half as much electricity again, every day. And every one of those watts ends up as heat in your tent.

Why the box can mislead ▶ 3:48

Many cheap lights are named after a wattage they never use. Growers report lights sold as 1,000 watts that draw about 100 from the wall. The big number can be what the chips could take at full power, or a claimed match for an old lamp.

The box may quote the chip, too. Today's best white LED chips are rated at just over 3 µmol/J. But that's the chip alone. A whole light loses some to its driver, its heat and its cover, so good fixtures land around 2.5 to 2.9.

Source · Kusuma, Pattison & Bugbee (2020); LED maker datasheets

So look for the actual power draw in the specs, or measure it with a plug-in power meter. That's the number that goes in the sum, and the one on your electricity bill.

When you compare lights, check four things:

• The real wattage.
• The efficiency, in µmol/J. If it isn't listed, divide the light's total output (its PPF) by its real watts.
• The maker's PPFD map, to see how evenly it covers a floor like yours.
• A dimmer, so you can set it exactly.

A PAR meter tells you what really reaches the plants.

Pick your light level, multiply by your floor, divide by the light's efficiency, and that's your real watts. Round up, then dim to suit, and ignore the number on the box.