The Best LED Light For Cannabis Plants

How To Choose The Best LED Light For Growing Cannabis Indoors

The most frequently asked question in cannabis today is: 

How Do I Choose The Best LED Light For Growing Cannabis Indoors?

Choosing the best LED light for growing cannabis involves a little science & math. LED lighting is cheap and very effective if it is set up correctly and the LED lights are of high quality.  Don’t skimp and don’t overpay; your lighting is a crucial element in growing cannabis. But choosing the best LED light for growing cannabis indoors can be tricky if you don’t understand some basic concepts about the amount of light cannabis plants need and how light intensity is measured.

To understand how to choose the best LED light for growing cannabis, we first need to talk about moles.  Not the little rodent that burrows, but moles as in the scientific unit of measure. There is a tiny bit of math involved, but nothing you and a calculator can’t handle.  

Cannabis Plants & Moles

What is a mole, and why is it important for cannabis plants?

A mole is a scientific measurement unit equal to 6.02 x 1023 particles.  That “particles” can be all sorts of things; atoms, molecules, electrons, and more.  For cannabis plants the particle we wish to measure is photons, or the amount/intensity of light that the plant is receiving.

A micromole is a unit of measure defined as 10-6 (one-millionth) of a mole. The symbol for micromole is commonly umol or μmol.  This is the unit of measure you see on LED lights and charts. By knowing the amount of umol a light produces, we can measure the amount of moles the plant receives during the day.

A mole is defined as approximately 6.022140857 x 1023, so therefore a micromole can also be defined as:

1 μmol = 6.022140857 x 1017

There are 86,400 seconds in a 24 hour day which tells us how many seconds are in each light schedule:

24/0 – 86,400 seconds

20/4 – 72,000 seconds

18/6 – 64,800 seconds

12/12 – 43,200 seconds

The ideal amount of light exposure for fruit or vegetable bearing plants, including cannabis, is between 20 and 40 moles of light per day.  Now we can work out how many umol your light must produce in order to provide 20 – 40 moles per day.

20 moles of light would be a minimum for an average harvest.

40 moles is the maximum amount for pushing plants and yields to the limit.

The optimal LED light output needed to meet minimum and maximum cannabis light requirements

24/0 Light Schedule:

20 moles = 232 umol

40 moles = 463 umol

20/4 Light Schedule:

20 moles = 278 umol

40 moles = 556 umol

18/6 Light Schedule:

20 moles = 309 umol

40 moles = 617 umol

12/12 Light Schedule:

20 moles = 463 umol

40 moles = 926 umol

Quick Fact:  The Sun produces between 900-1500 umol

During flowering phase at 12/12 you need a stronger light or you need to move the light closer to the plants. Moving the light closer to the plants decreases the radius of the area the light will cover plan to buy a light that can produce enough light during a 12/12 light schedule if you plan to grow photo plants indoors. Most LED lights come with a chart showing the umol produced at certain heights, and the size of the area covered by that height.  Here is an example of one from a 2000w led(100 x 10w).

Let’s interpret this diagram:

If we were to hang this light 18 inches above the plants, it would produce 865 umol, over a circular area with a radius of 45cm(18 inches).  Thus it will illuminate a circle with a 3 foot diameter( and honestly these shapes are more like ovals since the lights are typically rectangular, not square or circular in form).  That is enough light to provide just under 40 moles of light during flowering (12/12) which requires 926 umol to provide the full 40 moles. So let’s say this light would provide about 37 moles or so under 12/12 lighting at a height of 18 inches.  Here is the actual math:

43,200 x 865 = 37,368,000 umol = 37.38 moles over a 3 ft diameter circular area

Let’s raise this same light to 24” above the plants.  Now it produces 508 umol over a 4 ft diameter circle(60cm = 2 feet).  On an 18/6 light schedule this would produce around 33 moles. Here is the math:

64,800 x 508 = 32,918,400 umol = 32.9 moles over a 4 ft diameter circular area.

PPFD Maps

The umol chart above is a bit simplistic and showing an average intensity throughout the circle. In truth, the light intensity throughout the circle, or oval, is not equal. The light given off by the LED is more intense directly below the lamp, and weakens as it spreads out towards the edges of the circle. That means plants along the sides of a tent get less light than plants in the middle. The light intensity would actually look something like this.

As this PPFD map shows, light will be weakest in the corners, and will typically have one axis that is slightly stronger than the other due to the shape and placement of the lights within the housing. Over a 4×4 area this light may average 580 umol, but as you can see that will not mean that every plant receives 580 umol. Plants in the middle right under the light may be receiving too much light, while plants in the corner are not receiving enough.

So as you can see, the same light can produce a varying amount of moles over different size areas dependent on its height from the plants.  By understanding umol and how to interpret these light charts, you can easily pick out the best LED light for your tent. Just be sure that the light and the distance from the plants works out such that your plants are receiving 20-40 moles worth of light every day.

Choosing the best LED light for growing cannabis also involves the manufacturer of the light.  Beware cheap LED lights as they may not be full spectrum, may not last as long as promised, and may provide umol charts and figures that are not entirely accurate in order to boost claims and sales.  Figure out the right wattage by doing the math and then buy from a trusted and well reviewed manufacturer, brand, and/or retailer.

Choosing the best LED light for your grow should be a very simple and straightforward process. You should also now be able to set a custom light schedule that provides the 20-40 moles of lights to your plants based on your LED light output and its distance from the plants. By providing the correct amount of light to your cannabis plants, you will grow healthier and higher yielding plants.