Grow light calculator — DLI, kWh and running cost for a microgreens setup
Updated: 2026-10-18 · ~5 min read
- Two inputs decide the plant result: PPFD (how strong the light is at canopy level) and hours. Multiplied they give DLI, which is what the crop actually uses.
- Two inputs decide the bill: real wattage of the fixture and your price per kWh. A two-tray shelf on 14 h is roughly 12–13 kWh a month.
- If the DLI comes out low, lengthen the day before buying a bigger lamp — published work matched yields at equal DLI with lower intensity and a longer photoperiod.
- Box wattage is marketing; what matters is watts per tray (roughly 5–15 W for microgreens) and distance to the canopy.
Calculator
— mol/m²/day — Daily Light Integral for the crop
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Energy: — kWh/day, — kWh per month. Running cost: — per month, — per tray-harvest.
What the calculator assumes
DLI = PPFD × hours × 0.0036. Worked example: 200 µmol/m²/s for 14 h = 200 × 14 × 0.0036 ≈ 10.1 mol/m²/day. That sits inside the range microgreens are happy with; the common CEA benchmark of ~14.4 mol/m²/day is reached at 250 µmol/m²/s for 16 h, or 170 µmol/m²/s for a full 24 h day.
Energy = watts × hours × 30 ÷ 1000. A 30 W fixture on 14 h gives 30 × 14 × 30 ÷ 1000 = 12.6 kWh per month. Whether that is cheap depends on the tariff, not on the lamp: at 0.16 per kWh the light bill is a bit over 2 units of currency per month — usually less than one bag of seed.
Cost per tray-harvest spreads that monthly figure over your cycle: monthly cost × (cycle days ÷ 30) ÷ number of trays.
Reading the result
| DLI result | What it means for microgreens | Fix |
|---|---|---|
| under 6 mol/m²/day | leggy stems, thin mats, slow brassicas; a sunny windowsill in winter sits here | bring the lamp closer first, then add hours |
| 6–15 mol/m²/day | the working range for radish, sunflower, pea, brassicas | nothing — hold the schedule and ventilate well |
| above 17–20 mol/m²/day | no yield gain for microgreens, more heat and more electricity | cut hours or dim; use the saved power on a weak fixture elsewhere |
On the fixture side: watts per tray is the practical sanity check. Home setups typically land at 5–15 W per tray. If the calculator shows 40 W per tray, either the lamp is oversized for the shelf or the tray count is wrong — and oversized lamps mostly add heat, not yield.
The one number the calculator cannot know is the PPFD your lamp actually delivers at your shelf height. A thin LED strip 5 cm above the tray can beat a 200 W panel hanging a metre up. Measure if you can (a quantum sensor, or a phone light-meter app used only for comparison), otherwise treat the PPFD field as a target you tune by plant response: stretched stems mean too little, bleached or drying leaf edges mean too close.
Why running longer beats buying a bigger lamp
In vertical-farming work on lettuce, the same DLI (~14.4 mol/m²/day) delivered as continuous light at 170 µmol/m²/s matched 16 h at 250 µmol/m²/s for yield and quality, with better energy performance. In a separate controlled-environment study on leafy greens, intensities of 150, 200, 250 and 300 µmol/m²/s were compared at equal DLI, and the spectrum (red–blue mix) mattered as much as intensity. Translation for a home grower: intensity and day length are interchangeable inside a range, so the cheapest fix for a weak lamp is more hours, not more watts.
Frequently asked questions
Do I need a grow light for microgreens at all?
Not always. A bright south-facing window can carry fast crops (radish, mustard) through summer. In winter the DLI falls far below what seedlings need, plants stretch and mould risk rises, so one small LED panel is the cheapest insurance in the whole setup.
How many watts per tray is normal?
Roughly 5–15 W of real draw per tray for microgreens. Two stacked trays under a 30 W quantum board sit at the upper end of that range and are already generous for most species.
Is 24/7 light wasteful?
Not for the plant: low-intensity continuous light delivered the same yield at equal DLI in published work. It is mostly inconvenient — you lose the nightly temperature dip, and inspection is easier with a dark block of 6–8 hours.
Does the calculator include fans, dehumidifiers or heating?
No, it prices lighting only. Air movement and winter heating usually cost more than the lamps; see the winter cost breakdown for a full-season estimate.
How accurate is the monthly cost?
Arithmetically exact for the watts and hours you enter, but tariff-dependent: household rates vary by a factor of three between regions and tariffs, and a standing charge is not included. Enter your own price per kWh from a recent bill.
Sources: PMC13164867 — spectrum and intensity (150–300 µmol/m²/s at a 16 h photoperiod) in leafy greens under controlled environment; PMC12995675 — equal DLI (~14.4 mol/m²/day) as 24 h at 170 µmol/m²/s vs 16 h at 250 µmol/m²/s in vertical-farming lettuce. Watts-per-tray, distance and symptom guidance are grower practice for home setups (October 2026). Related: LED lighting 101, watering and mould prevention, winter growing costs, days-to-harvest chart.