Power, spectrum, intensity: lighting decides a good part of your harvest in indoor growing, and it's marketing's favourite hunting ground. Here you'll learn to read a spec sheet like a pro, to size your LED to the watt, then to set height, intensity and photoperiod for light-hungry tomatoes, chillies and peppers. By the end, you'll know how many real watts to install above your tent, and above all why.
Real watts, not marketing watts
A lamp sold as "1000 W" often draws only 100 to 250 W at the socket. "Equivalent power" has no standardised definition, so no serious comparison is possible. Only three metrics are reliable: the real draw at the socket ("actual power draw"), the PPF (the total flux of usable photons, in µmol/s) and the PPFD maps measured at a stated height.
For the same output, a LED consumes 50 to 75 % less than an HPS: the gap on the bill alone justifies choosing LED.
You then size to the area, not to the volume. In fruiting, count 30 to 40 real W per square foot (≈ 320 to 430 W/m²), half that in vegetative growth; for seedlings and cuttings, 15 to 20 W/sq ft is enough (rule of thumb).
This rule holds for a fixture of average efficiency (≈ 2.3 to 2.5 µmol/J): recent, very efficient panels sometimes hit the target with 20 to 25 W/sq ft. It's efficiency that makes the watts vary, and we'll come back to that.
In the field, an undersized lamp "works" in vegetative growth, then hits a ceiling in fruiting: the well-covered zone shrinks, the plants on the edges stretch towards the centre, and you end up rotating your pots every day.
🍅 Example: a 120 × 120 cm tent (1.44 m²), like the one chosen in the Getting started step, needs 465 to 620 real W at full fruiting; an 80 × 80 cm one (0.64 m²) makes do with 200 to 275 W.
Size your LED to the area, in real watts: 30 to 40 W/sq ft in fruiting (≈ 320 to 430 W/m²), half that in vegetative growth. Never to the volume, never to the "equivalent" power on the box.
Buying a lamp advertised at "1000 W" that only draws 100 real W. Those "HPS equivalent" watts aren't plugged into anything: check the real draw in the specifications, or measure it with a plug-in power meter (rule of thumb).
One last word about the "no-name" lamps flooding online marketplaces: advertised at 1000 W for a few tens of euros, they often draw less than 100, with an inflated PPF and PPFD maps copied from elsewhere. The worst part is the beginner who gets nothing under such a panel: they conclude that "LED doesn't work", when it's their lamp that's lying. And it doesn't stop there. Neither efficient, nor durable, nor safe, these fixtures end up in landfill after one season, and it's the community that pays to recycle a product that should never have been sold.
This subject is especially close to my heart. In 2009, an HPS lamp started a fire at my home, and it was that event that pushed me, at 19, to found GrowLED. Ever since, I've been uncompromising about the quality of the fixtures we select: PPE measured at the socket, real PPFD maps, a solid warranty. So when I see those no-name panels inflating their figures and ending up in landfill after one season, it makes me angry, because it's your trust and your safety they're damaging. Take the time to check the measurements before you buy: your harvest and your peace of mind depend on it.
Joseph, founder of GrowLED
