The Grow Light Numbers That Actually Matter (Watts, PPFD)

3 min read

A grow light spec sheet showing PPFD map, wall draw and efficiency figures

Grow-light shopping is a minefield of inflated wattages and meaningless claims. But there are only four numbers you actually need to understand a light, and once you know them, the “1000W for sixty euro” lights expose themselves instantly. Here they are, in plain English.

The short version:

  • PPFD — light intensity at the canopy: veg 300–600, flower 600–900
  • DLI — the daily light total: veg ~20–40, flower ~40–60 mol/m²/day
  • Wall draw — the real watts from the socket (your bill and your heat)
  • Efficiency (µmol/J) — usable light per unit of electricity; good LEDs hit 2.5–3.0
  • No PPFD map or efficiency figure on the spec sheet = walk away

Want the full breakdown? Keep scrolling.

PPFD and DLI — how much light, and how much per day

PPFD is the intensity — how many usable photons hit a square metre of canopy each second. The targets: roughly 300–600 in veg and 600–900 in flower. Below 200 you’re growing noodles; above 900 you risk light stress without added CO2. My old blurple delivered maybe 250 in the centre and 80 at the edges — hungry across the whole canopy. If your light dims (most LEDs do), use it: seedlings want 300–400, not full power, so start around 50% and raise it through the grow. DLI is the daily total — PPFD multiplied out over the hours the light’s on — and you want roughly 20–40 mol/m²/day in veg, 40–60 in flower. You don’t hit these perfectly; they’re the range where the plant gets enough light to do its job without being starved or fried.

Wall draw and efficiency — what you actually pay for

Wall draw is the number that lands on your electricity bill: the watts the light actually pulls from the socket, not the marketing figure. My blurple was sold as “1000W” and drew 130W; a decent bar sold as 240W draws 240W, because honest makers don’t need to inflate. Wall draw tells you running cost, heat added to the tent, and (with efficiency) rough output. Efficiency — µmol/J — is how much usable light you get per unit of electricity, and it’s what separates a good light from an expensive space heater: a good 2026 LED runs 2.5–3.0 µmol/J, a decent HPS 1.7–1.9, so the LED makes ~50% more light per watt you pay for. If a spec sheet doesn’t list efficiency — like my blurple’s didn’t — the maker either doesn’t know or doesn’t want you to.

How do I use these to buy a light?

Demand the PPFD map — a grid of readings across an area at a stated height. No map is a red flag: they’re hiding bad numbers or haven’t tested it. Check the map covers your tent’s footprint at a sensible height (a great centre reading at 30cm is useless if the corners read 200). Confirm the real wall draw matches what’s claimed, and that the efficiency is listed and competitive. Match the light to the tent — a 200W LED for an 80x80cm, 300–320W for a 120x120cm — and you’ll never buy a dud. DIG stock lights that publish honest maps and figures, which is half the battle.

FAQ

What PPFD does cannabis need? Roughly 300–600 in veg and 600–900 in flower at the canopy. Below 200 gives weak, stretchy growth; above 900 risks light stress without supplemental CO2.

What’s the difference between marketing watts and wall draw? Marketing watts are often an inflated or theoretical figure; wall draw is the real power pulled from the socket. Only wall draw tells you running cost, heat and rough output.

What’s a good grow light efficiency? For a 2026 LED, about 2.5–3.0 µmol/J. If a light doesn’t publish its efficiency or a PPFD map, treat that as a reason to avoid it.

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