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Photos and Autos | The Recommended PAR levels for Photos and Autos

LED grow lights allow growers to push for higher growth rates

Growers have been increasing the light intensity or PAR intensity in the grow room thanks to the increased efficiency of LED grow lights. Increasing efficiency means the fixtures run cooler and allow the use of higher wattage LED grow lights in grow tents than were used before.

Recommended PAR intensity for photoperiod plants is increasing

Growers are looking to increase the PAR intensity in their grow rooms. Grow light manufacturers are now recommending about 800µmols/m²/second average PAR. This is the level with the best return in terms of growth rate per watt. You can increase PAR intensity up to 1,500 µmols/m²/second without increasing CO2 and still get increases in grow rate and yield. However the rate of return reduces and the system is less efficient.

PAR intensity vs growth rate for photoperiod plants on a 12 hour flowering light cycle

PAR intensity levels of 1,000 µmols/m²/second and greater require raised CO2 levels which require closed systems and expensive CO2 cannisters, sensors and controls.

Higher PAR intensity above 1,000 µmols required elevated CO2

Recommended PAR intensity for photoperiod plants

For the novice to experienced grower applying PAR intensity levels of about 800 to 900 µmols/m²/second for Photoperiod plants will achieve very good yields from your grow space. To calculate what this means in terms of grow light wattage here is a table to convert this PAR intensity to consumed power for different types of grow light systems:

Photoperiod plants grow light wattage

Light period 12 hours
Grow light system System Efficiency (µmols/watt) Target average PAR µmols/m² DLI
(12 hours)
Watts/m² Watts/sq.ft
White & RED LEDs 2.4 900 40 375 30
Old Blurple LED
(over four years old)
1.4 900 40 650 50
High Pressure Sodium 1.4 900 40 650 50
Fluorescent 0.7 900 40 1300 100

Maximising the Daily Light Integral DLI

The average PAR intensity of 900 µmols/m²/second over the 12 hour period equates to about 40 DLI (Daily Light Integral, the amount of PAR reaching the plant over the total light cycle).

This is an important measure because, although plants can absorb PAR intensity up to at least 1,500 µmols/m²/second for a period of time, plants are limited as to how much light they can absorb over the total light cycle. 40 DLI is generally accepted as the upper limit for plants to absorb in one day.

Recommended PAR intensity for auto flowering plants is lower

Autoflowering plants do not require switching to a 12 hour light cycle to stimulate the plants to flower. For Auto flowering plants the typical light cycle is 20 hours on and 4 hours off. This maximises the time the lights are on while still allowing a few hours darkness for the plant to 'sleep' and metabolise.

Because the day is longer the PAR intensity can be lower to reach the same maximum 40 DLI. Here is the table for the recommended grow light system wattage to achieve our recommended PAR intensity of 550 µmols/m²/second for Autoflowering plants.

Autoperiod plants grow light wattage

Light period 20 hours

Grow light system System Efficiency (µmols/watt) Target average PAR µmols/m² DLI
(20 hours)
Watts/m² Watts/sq.ft
White & RED LEDs 2.4 550 40 230 20
Old Blurple LED
(over four years old)
1.4 550 40 400 35
High Pressure Sodium 1.4 550 40 400 35
Fluorescent 0.7 550 40 800 75


There are caveats to consider of course.

If you go over the maximum I have recommended in these tables you should still get an increase in growth rate, it just may not be a '1 for 1' increase i.e. 10% more PAR intensity = 10% more growth, for example it may be 5% more growth for 10% more PAR intensity

It is strain dependent, some are much more suitable to high light intensity than others

Grow room conditions such as temperature, nutrients, humidity, CO2 levels etc can restrict the growth potential from your plants if not optimised



3 thoughts on “Photos and Autos | The Recommended PAR levels for Photos and Autos

James Tillson

I have a migro aray 4 in a 2″×4″ room. Please advise as to height and intensity for seedlings.


February 14, 2024 at 13:55pm
Partech LED Ltd

Hi, 2 x ARAY 8 will suit your space perfectly

June 13, 2023 at 13:58pm
Jeff compo

Hi I’m starting a grow in a tent that 4 ft wide 8ft long..I was wondering your recommendation on a light and/or the amount of light needed for 6 plants. Im relatively new but not completely inexperienced. Im still using soil and advanced nutrients on some and no nutrients on the others. Thanks for any advice you have.

March 29, 2022 at 13:49pm

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