September 08, 2022

What size grow light do I need?

By Shane Torpey
What size grow light do I need?

The units we use to measure the light intensity for growing plants are called PAR (Photosynthetically Active Radiation) and are measured in µmols/m²/second. A wide range of PAR intensity can be used to grow plants indoors and there are positives and negatives in using either low or high PAR intensity.

Recommended PAR intensity for all stages of growth

Young plants from seedling to a few weeks old generally require lower PAR intensity of about 150 to 250 µmols/m²/second as they can be stressed if the light intensity is too high. From two or three weeks old plants can utilise much higher PAR intensity and it can be increased over week or two to about 800 µmols/m²/second for the VEG stage and into the Flowering stage.

In late Veg and flowering you can use PAR intensity from about 300 to 1,500 µmols/m²/second and the growth rate will vary accordingly.


However the green line represents growing in a normal environment without enhanced CO2 levels and at PAR intensity of about 800 µmols/m²/second the rate of increase in growth reduces as the PAR intensity increases. This is why most grow light manufacturers recommend this level for their fixtures.

Is it worth using CO2 in a closed system grow room?

If you run a closed system i.e. run CO2 and Air conditioning together to maintain artificially high levels of CO2 in the grow room, then the plants can utilise the higher PAR intensity more efficiently and growth increases at a higher rate above 800 µmols/m²/second. It is recommended to run the CO2 levels to the same parts per million (PPM) as the PAR intensity. For example at 1,200 µmols/m²/second you should increase CO2 levels to 1,200 ppm (normal atmospheric levels are 400ppm).

However this system is expensive and complex to run. The CO2 cylinders are heavy, hard to source and expensive. You also need CO2 regulators, CO2 controllers and alarms to protect you from CO2 poisoning. The total CO2 system could cost $1,000 to $2,000 to setup. You would also have to supply an split AC system to control the temperature and humidity in the grow room (you cannot use extract fans as they will remove the CO2 and it will not build up to the required levels.

If you are very limited for grow space and an expert grower this may make sense to implement but if you wish to increase your growing capacity a much simpler option is to add more area to your grow room or get a bigger or additional grow tent.

Grow Light Wattage

To achieve the most efficient PAR intensity for growing of 800 µmols/m²/second with new LED grow lights it is necessary to size your grow light wattage as follows.

Recommended PAR intensity for Autoflowers

The light cycle for photoperiod plants is 12 hours per day in the flowering stage whereas the light cycle for autoflower plants is up to 24 hours per day. This means up to twice as much PAR can be delivered to autoflowers than photoperiod plants for the same PAR intensity. To allow for this we recommend a lower PAR intensity or 550 µmols/m²/second for autoflower grows.



Category_Grow lighting tips grow room setup