How Big A Grow Light Do I Need?
We show you how to convert from the light intensity you want in PAR to a grow light size in watts.
You can use this calculator for any grow light system and any size grow area.
How to to calculate the wattage of your grow light in four steps
Note: There is a detailed explanation of the calculator underneath
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Explanation of the Grow Light sizing calculator
Watts per metre
Old methods of calculating the size of your grow light used the units watts per metre. These calculations were based around Metal Halide or High Pressure sodium bulbs and are not useful for comparing the wide range of technologies available today.
We no longer specify lighting for plants in terms of lumens. Lumens are units measuring how bright humans perceive light and not plants (Lumens are for Humans!).
Photosynthetically Active Radiation (PAR)
The industry is now adapting fast to the most accurate and useful measure for plants which is Photosynthetically Active Radiation or PAR. PAR is a measure of the energy emitted from a light source that is used by plants for photosynthesis and growth.
If you are not familiar with PAR or need a refresher we have a detailed guide HERE on all of the relevant horticultural lighting definitions and terms.
What’s the difference between PAR, PPF & PPFD? A brief overview..
PAR or Photosynthetically active radiation is a general term for the light plants use for photosynthesis. It is the range of light including Blue, Green and Red and ranges from wavelengths of 400nm to 700nm. When measuring in PAR all colours or wavelengths of light are deemed to photosynthesise at the same rate.
Photosynthetic Photon Flux is the measure of how much PAR a grow light emits. A published PPF figure for a light grow light is not the amount of PAR that will reach your grow area. The PPF from a light bulb and reflector in a grow tent will typically reduce by at least 25% by the time it reaches your plants. This reduction is due to light escaping (going out of the grow area) or absorbed by the reflector and reflective walls (no reflective surface is 100% reflective).
For example: The PPF for a 600 watt High Pressure Sodium bulb is about 1,100 umols.
In a very efficiency setup with a large high quality reflector, highly reflective walls and at optimum hanging height you will lose 25% of the light. So you will get an average PPFD on a one metre square area of about 800 umols.
In a low efficiency setup such as a ‘Barn’ reflector, budget tent with reflective walls and a hanging height 10cm above optimum you will lose 40% of the light to reflective losses. So you will get a PPFD on a one metre square area of about 660 umols.
PPFD or Photosynthetic Photon Flux Density is the PAR measurement for a grow area & is measured in units called umoles/m2/second (micromoles per metre squared per second). This is how much PAR is landing on one metre square area. PPFD is the most important number for a grow light system.
The best way to show this output for a grow light is to publish the PPFD over the specified grow area for the grow light system. An example is shown on the left.
Specifying the PPFD as a single point under the grow light is not sufficient. Why? Because that is where the PPFD is highest and it will reduce as you move away from the centre to the edges of the grow area. The average PPFD across your grow area will be much less than the centre point measurement.