Grow Light Calculator

Planting area, m²

What you are growing

Photoperiod, h

Fixture efficacy, µmol/J

Price of a kilowatt-hour

Result

Fixture power, W48
PPFD needed, µmol/m²·s238
Total PPF, µmol/s119
Target DLI, mol/m²·day12
Use a month, kWh20
Cost a month$110.00

Plants count light per day, not per watt

Calculates the grow light wattage a planting area needs from the daily light integral, the photoperiod and the fixture efficacy.

Watts describe the lamp, not the light. What a plant responds to is PPFD — the rate of usable photons landing on a leaf — and over a day that adds up to the daily light integral, the DLI.

Seedlings want about 12 mol a square metre a day, leafy greens 15, fruiting plants 30. Fix the DLI and the photoperiod and the required PPFD falls out; divide by the fixture's efficacy and you have the wattage.

What this calculator shows

The wattage of LED needed over the area
The PPFD and the total photon flux behind it
Monthly consumption and cost

What to keep in mind

Fixture efficacy defaults to 2.5 µmol per joule, which is typical of a decent modern LED. Cheap fixtures manage 1.5 and the best exceed 3.
All the light is assumed to land on the plants. In practice a bare fixture wastes a good share to the sides, which a reflector recovers.

FAQs

What is DLI?

The daily light integral: the total usable light a plant receives over a day, in moles of photons per square metre. It is the figure that predicts growth, not wattage.

How many hours should seedlings be lit?

Fourteen to sixteen. Plants also need darkness for respiration, so twenty-four-hour lighting is not a shortcut.

Can a normal LED bulb be used?

It will grow something, but the efficacy is far lower and much of the light is in wavelengths plants use poorly. The wattage figure here assumes a horticultural fixture.

Does distance from the plants matter?

Very much. PPFD falls off roughly with the square of the distance, so the same fixture twice as far away delivers about a quarter of the light.

Worked example

Half a square metre of seedlings

Input: 0.5 m², seedlings, a 14-hour day, 2.5 µmol/J

Output: about 48 W

Note: That comes from a target of 12 mol a day, which needs a PPFD of 238 over fourteen hours. Stretch the day to sixteen hours and the same DLI needs only 208 — and about 42 W.

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Grow Light Calculator

Result

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