Calculator · Light

Sun, window, lamp: the right dose of light

The calculator estimates the light your plants receive where they grow, month by month, using actual sunshine data for Belgium. Then it sizes the LED supplement: target light, wattage, hours on and cost.

Location
Plants
The lamp

Measure before you buy.

1 Where your plants are

I measureTwo lux readings, indoors and outdoors Sunroom, greenhouse≈ 60 % of outdoor light I know my %Measured another time No daylightBasement, closet, windowless room

Measure it in 2 minutes with a free “light meter” app

One day evenly covered, or the sun hidden, around noon.
Put the phone down instead of the plants, sensor facing up: write down the lux.
Right after, same step outdoors, in the open (balcony, garden): write down the lux.
The ratio of the two tells you what share of outdoor light reaches your plants. Apps aren’t accurate in absolute value, but the same phone gives a reliable ratio.

Lux where the plants are

lux

Lux outdoors, at the same time

lux

2 Your plants and your season

Seeds, cuttings5 to 10 mol/m²/day Salad greens, lettuce12 to 17 mol/m²/day Herbs (basil…)15 to 25 mol/m²/day Tomatoes, peppers, cucumbers20 to 30 mol/m²/day

I grow from

àAll year: January to December

natural light at the spot supplemental LED minimum target

3 The supplemental lamp

Lit area, width

Depth

Maximum on-time

h / day

14 to 18 h (UNH)

Lamp efficiency

µmol/J

Spec sheet; 2.5 to 2.8 for the best LEDs (Kusuma, 2020)

Price per kWhSelectra, September 2026

My lamp only lists lumens

Rated luminous flux

Rough figure for a “daylight” white light: about 17 µmol/s per 1000 lm (Wikipedia, photosynthetically active radiation). Lumens measure what the human eye sees, not what the plant uses: choose a lamp that states its PPF or PPFD.

Your result…

Natural light: normal global radiation at Uccle, RMI, 1991-2020, converted to plant-usable light at about 2 mol per MJ (published range 1.86 to 2.31; BioNumbers). Light targets: Virginia Cooperative Extension (SPES-720) and our crop guides; sunroom: 30 to 50 % losses, a Virginia Tech benchmark for greenhouses. Lux to PPFD conversion for daylight: 0.0185 (Apogee Instruments). These are monthly averages: a gray week in November is much darker. Measure, then adjust.