Outdoors the lighting question is already solved: full sun delivers 1,800–2,200 µmol/m²/s, more than any LED you will ever buy. Indoors is where the trouble starts. Room lighting that looks perfectly bright to you is near darkness for a plant — on a table under a normal ceiling lamp you measure 10–30 µmol/m²/s. That is roughly one percent of full sun, which is exactly why plants on a living room shelf go pale and leggy.
The four terms you actually need
| Term | What it measures | Why it matters |
|---|---|---|
| PAR | The 400–700 nm waveband plants use for photosynthesis | Not a number but a range; everything else is measured inside it |
| PPFD | Photons per second per square metre (µmol/m²/s) | Light intensity at the leaf, not at the lamp |
| DLI | Total daily photon dose (mol/m²/day) | The only number that genuinely predicts growth |
| Kelvin (K) | Light colour: 3000 K warm, 6500 K cool | Rough spectrum guidance, tells you nothing about intensity |
| Lumen | Light weighted by human eye sensitivity | Misleading for plants — see right below |
Why lumens mislead you
The lumen was invented for humans. It is weighted by the eye sensitivity curve, which peaks in the green (555 nm) — and green is exactly what plants reflect most, which is why leaves look green. Photosynthesis mainly uses blue around 450 nm and red around 660 nm, precisely what the lumen barely counts. A lamp rated at “10,000 lumens” can therefore produce less growth than a 6,000 lumen lamp with a different spectrum.
When buying, look at µmol/J (efficacy). Cheap marketplace LED panels manage 1.0–1.6 µmol/J, solid grow LEDs 2.3–2.8, top units 3.0–3.5. The gap between 1.3 and 2.6 means half the electricity bill for the same growth. If a manufacturer states neither µmol/J nor PPFD at a defined distance, you have nothing to compare.
DLI: the calculation you can do in your head
The formula is simple: DLI = PPFD × hours × 0.0036. Example: 200 µmol/m²/s for 14 hours gives 200 × 14 × 0.0036 = 10.1 mol/m²/day. Slightly too little for lettuce — so either extend to 16 hours (11.5) or push to 250 µmol (14.4).
| Crop | PPFD (µmol/m²/s) | Hours/day | Target DLI |
|---|---|---|---|
| Seedlings and microgreens | 100–150 | 14–16 | 6–10 |
| Lettuce, lamb's lettuce | 150–250 | 14–16 | 12–17 |
| Rocket, spinach, chard | 150–250 | 12–14 | 10–16 |
| Basil, parsley, dill | 200–300 | 14–16 | 14–20 |
| Strawberries | 250–350 | 12–14 | 17–22 |
| Tomato, pepper, chilli | 300–500 | 12–16 | 20–30 |
Above the target DLI plants do not grow faster. They spend the energy on defence instead: leaves bleach, lettuce margins turn brown (tipburn), and some cultivars go reddish-purple from anthocyanins. More light without more nutrients and enough water always ends worse than moderate light.
Distance and light distribution
Intensity drops with the square of distance: double the gap and only a quarter of the light reaches the leaf. With a small lamp 10 cm changes everything; with a large panel it barely registers.
- Leafy greens and herbs: 20–40 cm above the canopy.
- Fruiting crops: 30–50 cm — they need more light and tolerate it.
- Too bright: the leaves nearest the lamp bleach, cup upwards or brown at the edges.
- Too dim: the stem stretches, the gaps between leaves grow long, the plant leans toward the source.
A vertical tower is a special case: 32 planting spots spread over 180 cm of height, so one ceiling lamp scorches the top plants and leaves the bottom ones in half-darkness. Outdoors the sun solves this by coming in from the side and shifting angle through the day. Indoors you need light standing upright beside the tower — two LED strips on both sides, or a single strip plus rotating the tower 90° every few days. More on the build itself in Vertical tower.
Full spectrum or “blurple”?
Purple lamps (blue and red diodes only, so-called blurple) were more efficient about a decade ago, because white LEDs were weak back then. Today good white LEDs with added 660 nm red diodes match or beat them — and you can actually see your plants. Under purple light you spot neither a yellowing leaf nor the first pests.
In practice: 3500–4000 K white with a red boost covers everything from germination to fruit. 6500 K carries more blue and produces compact, stocky growth — good for propagation. 2700–3000 K carries more red and favours flowering and fruit set.
What it costs in electricity
The maths: kWh per month = watts × hours per day × 30 ÷ 1000. Croatian electricity runs about 0.15 €/kWh, German about 0.35 €/kWh.
| Setup | Power | Hours/day | kWh/month | Cost at 0.15 € | Cost at 0.35 € |
|---|---|---|---|---|---|
| LED strip for 2–3 plants | 20 W | 14 | 8.4 | ~1.30 € | ~2.90 € |
| LED panel for an indoor tower | 60 W | 14 | 25.2 | ~3.80 € | ~8.80 € |
| Tower pump (interval) | 8 W | 12 equiv. | 2.9 | ~0.45 € | ~1.00 € |
Being honest about it: growing lettuce under artificial light is not cheaper than buying lettuce, especially at northern European electricity prices. It pays off for freshness, for herbs that otherwise rot half-used in the fridge, and for control over what ends up on the leaf. Outdoors, where the sun does the lighting, you only pay for the pump — and the whole calculation changes.
Duration: 12 to 16 hours, not 24
Most crops want 12–16 hours of light. Lettuce tolerates 18–20 hours and grows faster on it, but sensitive cultivars respond with brown leaf margins. Tomato is the clear exception: under continuous light with no dark phase it develops chlorosis and yellows. Fit a timer and keep the same schedule daily — plants respond to rhythm, not only to quantity.
One more reason for a timer and a well-placed lamp: light falling on the reservoir feeds algae. If your tank is translucent, wrap it or keep it in the lamp's shadow.