Grow Light Calculator

Written byOleh Soloninupdated 25 September 2026

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$3.60

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.

The calculator sets targets of 12 mol a square metre a day for seedlings, 15 for leafy greens and 30 for fruiting plants; a real crop's need depends on the variety and the stage. Fix the DLI and the photoperiod and the required PPFD falls out; divide by the fixture's efficacy and you have the wattage.

How to use

Enter the growing area — the shelf, rack or windowsill the light has to cover.
Choose what you are growing: it sets the daily light integral, the DLI, the plants need.
Enter the photoperiod in hours.
Enter the fixture efficacy in µmol/J from its specification and your electricity price — the result gives the wattage and the running cost.

How much light different plants need

A plant counts the day's total light, not the watts. Fruiting plants need nearly four times the light of houseplants: tomatoes or peppers under a lamp take 119 W per half square metre, not 32.

Every row comes from the calculator on this page for 0.5 m², a 14-hour day and a fixture efficacy of 2.5 µmol/J.
Growing	DLI, mol/m²/day	PPFD over 14 h	W per 0.5 m²	kWh a month	A month at 0.18 per kWh
Houseplants	8	159	32	13.3	2.40
Seedlings	12	238	48	20	3.60
Leafy greens	15	298	60	25	4.50
Flowering plants	25	496	99	41.7	7.50
Fruiting plants	30	595	119	50	9.00

A longer day buys a smaller lamp, not cheaper light

For the same daily light integral, a longer day lets you use a weaker fixture — but the energy is exactly the same: the plants receive the same moles of photons, and the fixture spends 20 kWh a month delivering them whatever the schedule.

At a given DLI only the fixture's efficacy changes the bill: at 3 µmol/J the same seedlings take 16.7 kWh a month, at 1.5 they take 33.3.

Seedlings, 0.5 m², 2.5 µmol/J. Every row comes from the calculator on this page.
Photoperiod, h	PPFD, µmol/m²/s	Wattage, W	kWh a month
12	278	56	20
14	238	48	20
16	208	42	20
18	185	37	20

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/J. Look for your fixture's own figure in its specification as PPE, photosynthetic photon efficacy: the lower it is, the more watts the same light takes.
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 watts of grow light per square metre of seedlings?

About 95 W over a 14-hour day at an efficacy of 2.5 µmol/J, or 48 W for half a square metre. Fruiting plants on a 16-hour day need 208 W per square metre.

How many hours should seedlings be lit?

It is a trade between wattage and time: half a square metre needs 42 W over 16 hours or 56 W over 12, for the same total light. Do not run the light round the clock — tomatoes, for one, develop leaf injury under continuous lighting.

What does it cost to light seedlings?

Half a square metre over a 14-hour day uses 20 kWh a month — 3.60 at 0.18 a kWh. A longer day does not change that; a more efficient fixture does: at 3 µmol/J it is 16.7 kWh.

Can a normal LED bulb be used?

It can be used, but lumens alone are not enough to calculate it. Household bulbs are sold by lumens, which are weighted to the human eye rather than to plants. This calculator needs the efficacy in µmol/J, which horticultural fixtures state.

Does the hanging height matter?

Very much. The higher the fixture, the wider the pool of light and the fewer photons land on each leaf, so the PPFD here has to be reached at leaf level. Check it against the maker's PPFD chart for your height or with a quantum sensor.

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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