Grow light calculator
Work out how much light your crop really needs, how big a fixture to hang, and what it costs to run, on the grid or fully off-grid. Built for every crop and every location FairGrow reaches.
From crop target to fixture and power bill
Plants do not care about watts, they care about light reaching the leaves over a day. This planner turns your crop, your growing area and your hours of light into a target PPFD, a fixture size and a clear running cost, including solar and battery sizing for remote sites with no mains power.
Your setup
Light sensors & water (PAR / lux control)
Fine-tune the fixture and power
Off-grid / remote site (solar + battery)
Your light plan
Running cost
Estimates use established horticultural DLI targets and standard LED efficiency figures. Real results depend on fixture quality, mounting height, reflectivity and how evenly the light spreads. Treat the numbers as a solid starting point, then confirm at the canopy with a light reading. FairGrow light sensors log actual PPFD and DLI in the field so you can fine-tune.
Light targets per crop
The defaults behind the calculator. DLI is the total daily light dose in mol/m²/day, PPFD is the intensity at the canopy in µmol/m²/s. Ranges are typical for healthy growth, not hard limits.
| Crop | Target DLI | PPFD range | Light hours | Notes |
|---|
Quick light conversions
Field light meters often read in lux. These helpers turn everyday numbers into the units that matter for plants.
Lux to PPFD
PPFD + hours to DLI
How the numbers are built up
Crops want a daily light dose, not a wattage
Photosynthesis adds up over the day. The Daily Light Integral (DLI) captures that: the total amount of growth-light a plant receives in 24 hours. Lettuce is happy around 14 to 16, a tomato wants more than 22, while a shade-loving houseplant gets by on 5 or 6. Start from the crop target, and the right intensity follows.
Intensity, area and hours set the fixture
Spread the daily dose over your light hours and you get the intensity the canopy needs (PPFD). Multiply by the lit area, allow for how much light actually lands on the plants, and you have the total output a fixture must deliver. Divide by the efficiency of modern LEDs and you land on a realistic wattage, not a guess from a shop label.
Remote sites without mains power
Many of the places FairGrow works have no grid to plug into. Switch on off-grid mode and the planner sizes the solar array and battery to keep the light running, using your local sun hours and the autonomy you want for cloudy spells. It is the same approach we use to keep sensors alive in the field, applied to lighting.
Sources behind the sensor and water logic
The sensor-control and water-use estimates lean on established horticultural research. The main references we used:
- Albright, L.D., Both, A.J. & Chiu, A.J. (2000). Controlling greenhouse light to a consistent daily integral. Transactions of the ASAE 43(2): 421-431. Foundation of daily-light-integral lighting control.
- van Iersel, M.W. & Gianino, D. (2017). An adaptive control approach for LED lights can reduce the energy costs of supplemental lighting in greenhouses. HortScience 52(1): 72-77.
- Harbick, K., Albright, L.D. & Mattson, N.S. (2016). Electrical savings comparison of supplemental lighting control systems in greenhouse environments. ASABE Annual International Meeting.
- Stanghellini, C. (1987). Transpiration of greenhouse crops: an aid to climate management. PhD thesis, Wageningen. Radiation-driven greenhouse transpiration model.
- Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. (1998). Crop evapotranspiration (FAO Irrigation and Drainage Paper 56). Latent heat of vaporisation (2.45 MJ/kg) and the Penman-Monteith framework.
- Katsoulas, N. & Stanghellini, C. (2019). Modelling crop transpiration in greenhouses: different models for different applications. Agronomy 9(7): 392.
- The role of radiation in the modelling of crop evapotranspiration from open field to indoor crops (2022). Agronomy 12(11): 2593. Evapotranspiration as a near-linear function of radiation.
- Supplemental greenhouse lighting increased the water use efficiency, crop growth and cutting production in Cannabis sativa (2024), via PubMed Central. Evapotranspiration rose about 1.8x with more light while water use per gram of biomass fell.
- McCree, K.J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology 9: 191-216. Basis for the PAR waveband and the ~4.6 micromol/J conversion.
The water figure is an energy-balance estimate: transpiration is approximated from the daily light dose using the latent heat of vaporisation and a heat-load factor for the spot. It is a planning guide, not a replacement for a soil-moisture or flow sensor in the field.
Want this matched to your real site?
Share your crop, location and growing space. We will help size the lighting and the sensors that verify it, in the EU on proven Zigbee hardware and beyond it on custom gear.
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