LED Grow Light FAQ: PPFD, DLI, Distance, and Choosing a Supplier

These are the questions we are actually asked, answered in as few words as the question deserves. Where a question deserves a full article rather than a paragraph, we link to it.

FAQ

What are the best grow lights that actually work?

The ones that deliver a measured PPFD, in writing, across the whole footprint — not just at the centre. Almost every disappointing grow light is not a bad spectrum; it is a fixture that hits 600 µmol/m²/s in one spot and 120 in the corners, with an average printed on the box that nobody measured. Start from our PPFD and DLI calculator and work backwards from the DLI your crop needs.

Is PPFD or DLI more important?

DLI, for almost every case. PPFD is instantaneous intensity; DLI is the total light a plant receives in a day, and it is what actually drives photosynthesis and yield. PPFD matters only in that a low value means you cannot reach your DLI target within a photoperiod your plants will tolerate. A plant given a very high PPFD for a short day receives less total light than one given a moderate PPFD for a long day — and usually does worse for it.

How do I calculate DLI for plants?

Multiply average PPFD by hours of light per day, then by 0.0036. So a canopy at 300 µmol/m²/s under 16 hours receives 300 × 16 × 0.0036 = 17.3 mol/m²/day. In reverse, the PPF you need for a target is Target DLI × Area (m²) ÷ (Hours × 0.0036). The calculator does both.

What is the ideal DLI for plant growth?

For most crops, 12–20 mol/m²/day. Seedlings and cuttings want 4–8, microgreens 8–12, lettuce and leafy greens 12–17, herbs 14–20, and fruiting crops such as pepper and tomato 20–30. The full crop table has the ranges and photoperiods.

How do I build a PPFD and DLI chart for my setup?

In three columns: footprint, watts, and resulting DLI. Pick your fixture, note its PPF from the spec sheet, divide by the covered area to get average PPFD, then apply your photoperiod. Our sizing guide walks through it at room scale, and the desk setup guide does it for a shelf.

How far should a grow light be from plants?

20–30 cm above the canopy for herbs and leafy greens, 15–25 cm for seedlings, 25–35 cm for dense lettuce. Mount height is the biggest free variable you have, because PPFD falls off roughly with the square of distance — doubling the distance cuts intensity to about a quarter. Bleached, papery leaves facing the lamp mean too close; long stretched stems mean too far.

Is grow light wattage a useful number?

Only as a proxy for cost and heat. Compare efficacy in µmol/J instead. A 50W fixture at 2.8 µmol/J puts more usable light on the plants than a 90W fixture at 1.4 µmol/J, while drawing less than half the power and generating half the heat. Modern quantum-BOARD fixtures sit at 2.4–3.2 µmol/J; anything quoted much above 3.5 deserves a request for a test report.

What spectrum should a grow light have?

Full-spectrum white with a tuned red boost is the current default for good reasons: it is efficient, it looks acceptable to a human in a living room, and it is defensible to buyers. Fixed-blend horticultural spectra (purple, or red-heavy) still work but are increasingly seen as dated. The genuinely useful custom parameter is dimmability — a dimmable fixture serves seedlings and mature plants from one SKU.

Do I need a light meter to set up a grow light?

For the average, no — that is arithmetic from the spec sheet. For uniformity, yes, or you will not notice that half your tray is dark until the plants tell you. Watch for pale leaves and stretched internodes at the edges of the footprint; that is the free version of the measurement.

Can I grow plants with a sunny window and no grow light?

Sometimes. A bright south- or west-facing windowsill can deliver 10,000–15,000 lux, which is enough for leafy herbs and greens. The problem is reliability rather than level: the light changes through the day, halves under cloud, and glass filters part of the useful spectrum. A windowsill-only setup usually works until about March.

Can I run a grow light 24 hours a day?

You can, and each 1 mol/m²/day of DLI then needs only about 7.7 µmol/m²/s of PPFD. But plants respire continuously and burn sugars whether or not they are photosynthesising, so the same DLI spread over 16 hours usually outperforms 24. It also doubles the electricity bill and the heat load.

What is the best grow light for a small space or desk?

The smallest dimmable fixture that covers your footprint evenly. In a desk or kitchen shelf, a bar form factor beats a square panel because it throws light sideways into the canopy instead of down onto one spot, and a 20–45W unit is almost always the right answer. Full details in the small-space setup guide.

Who is the best LED grow light manufacturer?

That is a question about your market, not about us. What matters is whether the supplier is a factory or a trading company, whether their PPF and efficacy claims survive an arithmetic check, whether their test reports are model-specific and legally usable under your brand, and whether they will answer a warranty question in year two. Our buyer checklist is written so you can run it against us as well.

What is the MOQ for OEM or private-label grow lights?

From 200 pcs per model, per model rather than per order. Custom spectrum, a bespoke housing, or a new certification each add their own one-off cost and lead time on top. See private label for how volume bands change unit cost, and our OEM process for the full timeline.

Do you need UL, CE, or DLC certification to sell grow lights?

What you need depends on where you sell. The EU generally expects CE (LVD, EMC, RoHS, and often EN 62471); the UK expects UKCA; the US expects UL or ETL for retail plus FCC for EMC, with DLC if you want utility rebates. A critical detail: reports are model-specific, and are often issued in the manufacturer’s name — which may not let you apply your own brand without re-testing. Settle that before you commit.

Where to go deeper

• PPFD & DLI calculator — work out your own numbers, no email required.
• Key factors in choosing grow lights — the non-luminous factors that decide a purchase.
• The complete indoor basil guide — one crop, end to end.
• Desk and small-space setup — if you are growing on a shelf.
• How to choose a grow light manufacturer — the supplier-side checklist.
• OEM / ODM services — if you are buying for a brand rather than for yourself.