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What size grow light do I need?

Grow light wattage calculation

To calculate what wattage grow light you need for your grow area it is essential to understand the principals of grow lighting.

Grow light output is measured in PAR (Photosynthetically Active Radiation)

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.

Photosynthetically Active Radiation is measured as PPFD

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.


PAR intensity vs growth rate

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).

CO2 sealed grow room is necessary to use very high PAR intensity

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 for Photoperiod plants

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 wattage per square foot or square meter for photoperiod plants

Grow light wattage for Autoflower plants

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.

recommended wattage per square foot or square meter for autoflowerplants




3 thoughts on “What size grow light do I need?

Irish mess head

A lot of good information on here. I am literally taking notes. I appreciate all this information. Because people talk about PAR intensity but you have the words and expressions set up with links so I can really educate myself.. hopefully you have an explanation and an example of the math formula you can use to figure out what? Why did you need. I have a 2×2×4 tent. I using a 2-ft boost, grow LED lights. It is 120 volts/ 60hz and 16 watts.. I don’t think it’s enough. I look forward to reading the rest of your website. Thanks again

January 9, 2024 at 14:39pm
Partech LED Ltd

Eric, I am going to post a guide to buying a grow light soon, thanks, Shane

June 13, 2023 at 13:58pm
Eric Raiche

Man, I wish I had realized what I truly needed in a led light and what I don’t need, and why. Yes you cover this in your videos. Maybe you can make a video explaining this, you don’t need that this is what it takes. Anyway love your work wish I new then what I know now.

June 13, 2023 at 12:59pm

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