How to Choose a Hydroponic Grow Light
PPFD, DLI, watts per square meter, and hang distance — the four numbers that decide whether your grow light is right. No brand names, just physics.
BY ROOTLESS FARM
Picking a grow light is the single biggest hardware decision in any indoor build. The wrong light wastes electricity for years; the right one pays for itself in yield. Four numbers decide the call: PPFD, DLI, W/m², and hang distance. Everything else is marketing.
PPFD vs DLI — the only specs that matter
PPFD (Photosynthetic Photon Flux Density) is how many useful photons hit one square meter of canopy per second, measured in μmol/m²/s. Think of it as instantaneous light intensity. [PPF-DLI-01]
DLI (Daily Light Integral) is total photons received per square meter per day, measured in mol/m²/day. It is PPFD multiplied by photoperiod and converted to moles.
Target DLI by crop:
- Lettuce, kale, herbs: 12–17 mol/m²/day
- Tomato, pepper, cucumber: 20–30 mol/m²/day
- Strawberry: 17–22 mol/m²/day
A fixture's spec sheet should publish a PPFD map at a known distance. If it doesn't, treat the wattage rating as fiction and move on.
LED vs HPS vs fluorescent
LED dominates new builds for one reason: photon efficacy. Modern quantum-board LEDs deliver 2.5–3.0 μmol per joule. That means roughly 40–50% less electricity for the same useful light versus older technology. Heat output is also lower, so room cooling drops with the bill.
HPS (high-pressure sodium) still produces ~1.7 μmol/J and floods a footprint cheaply, but it runs hot, demands tall hang distances, degrades 10–15% per year, and needs ballast replacements. It survives in commercial greenhouses that already need the heat.
Fluorescent (T5, CFL) is fine for seedlings, clones, and short leafy greens within 20 cm of the bulb. Beyond that, photon density collapses. Skip fluorescents for anything taller than basil.
Wattage per square meter — sanity check the marketing
Watts measure electricity drawn, not light produced. A "1000W" fixture pulling 240 W from the wall is selling a story, not a spec. Use real wall-pull wattage and compare to crop needs:
| Crop class | Real W/m² (efficient LED) | Real W/m² (HPS) |
|---|---|---|
| Microgreens, seedlings | 80–120 | 150–200 |
| Lettuce, leafy herbs | 150–250 | 250–400 |
| Strawberry, cannabis veg | 250–400 | 400–600 |
| Fruiting (tomato, pepper) | 400–600 | 600–900 |
If a 60×60 cm tent claims a 1000 W LED, check pull wattage and photon map before believing the label. [CORN-CEA-01]
Hang distance and footprint
Light intensity falls off roughly with the inverse square of distance from the source. Doubling hang height quarters the PPFD on the canopy directly below. Manufacturers publish PPFD maps at 30, 45, and 60 cm — use them.
Rules of thumb for bar-style LEDs:
- Veg / leafy: 45–60 cm above canopy
- Flower / fruiting: 30–45 cm above canopy
- Adjust by leaf signal: curled, bleached tips = too close. Stretching, pale internodes = too far.
Single-point COB lights and HPS bulbs have a hot center and dim corners. Bar lights distribute photons more evenly. For a square footprint, prefer multi-bar fixtures over single-point sources.
Full-spectrum white vs blurple
Old blurple LEDs (heavy on 450 nm blue and 660 nm red) score well on μmol-per-dollar but make plants look gray, hide disease, and strain the grower's eyes. Modern full-spectrum white LEDs add green and far-red wavelengths that improve canopy penetration and let you see what's happening. [OSU-NUT-01]
For any space you spend time in, choose full-spectrum white at 3000–4000 K with a small red-channel boost. For sealed flowering tents you check briefly, blurple still saves money per μmol.
The buyer's checklist
- Does the spec sheet publish a PPFD map at canopy height? If no, walk away.
- Is the photon efficacy ≥ 2.3 μmol/J? Below that, you're heating the room, not feeding plants.
- Is the footprint matched to your tent or rack? An oversized fixture wastes photons on walls.
- Does the driver support dimming? You will want to dim during seedling and late flower.
- What is the warranty? Five years is the new floor for serious LED brands.
Buy on photons per joule and PPFD on canopy. Ignore the wattage on the box.
FAQ
5 entries- Q01How many watts per square meter do I need?
- For leafy greens, 150–250 W/m² of efficient white LED is enough. Fruiting crops want 400–600 W/m². HPS needs roughly 50% more wattage for the same useful photon output because it dumps more energy as heat.
- Q02PPFD or DLI — which spec should I shop by?
- Shop by PPFD at your canopy height, then verify DLI matches your crop. A fixture advertising "1000W equivalent" tells you nothing. A photon map showing 600 μmol/m²/s at 30 cm tells you everything.
- Q03Is full-spectrum white LED really better than blurple?
- For human-friendly grow rooms, yes — defects are visible, plants look like plants, and modern white LEDs match or beat blurple efficiency. Blurple still wins on raw μmol per dollar for closed tents you never enter.
- Q04How far should an LED hang from the canopy?
- For a 300–400 W bar light, start at 45–60 cm above the canopy during veg and 30–45 cm during flower. Tune by leaf reaction — curled tips means too close, stretching means too far.
- Q05Do I need supplemental UV or far-red?
- For lettuce, basil, and most herbs, no. For flowering or fruiting crops, a small far-red channel (730 nm) can shorten flower time by a few days. UV-A may improve compound density but adds cost and skin risk for the grower.