What Size LED Grow Light Do You Need? - Happy Hydro

What size LED grow light do you need for growing cannabis indoors? Unlike the world of Metal Halide (MH) and High-Pressure Sodium (HPS) lighting, LED lighting poses a bit of a mathematical challenge for rookie growers setting up their indoor grow space.

Wattage was the conventional approach to calculating the number of grow lights in a grow tent, but it doesn't easily convert into LEDs' language. When it comes to LED grow lights, wattage is not always a great conversion. 

Instead of strictly relying on wattage, you'll need to look at a few factors to make an accurate estimate for the number of LED grow lights you'll need and the appropriate size. 

Let's go over the following factors for determining effective grow light number and size:

  • Wattage
  • PPF
  • PAR Map

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The Easy Estimate on LED Grow Light Size

Thankfully, most grow tents come pre-fabricated in standard sizing. Presumably, the first step to preparing an indoor grow is ascertaining grow tent size. 

Are you working with a small 2 ft x 4 ft AC Infinity Grow Tent or a much larger 10 ft x 10 ft option, or something in between? 

In most situations, the grow tent decision comes first, which then influences the grow light decision. The smaller the tent, the easier the decision. Small tents can fit at most two LED grow lights. LED grow lights also come with helpful estimates for canopy coverage, in both veg and bloom. Put these two estimates together, and you have an easy formula for what size grow light you need.

For example, with smaller tents (4 ft x 4 ft and under), you can get away with a single LED grow light in most cases. Especially grow lights with impressive PPF output, like the 1700 μmol total output from then Grower's Choice ROI-E680 LED Grow Light. Super small tents can even get away with a Spider Farmer SF-1000 with 829 µmol total PPF.

Using HPS Equivalency and Wattage to Calculate LED Grow Light Requirements

If you are working with a larger grow tent or want to adopt more technical approaches, let's add in a wattage (HPS Equivalency) calculation. This calculation comes from HPS and MH lights, which have known conversions for wattage to canopy square footage.

LEDs should always come with an HPS equivalency in watts. However, wattage isn't as accurate when it comes to LEDs because wattage describes the power drawn from the wall, not the output. Remember, LEDs offer significant power savings over conventional lighting, and therefore using the wattage as a predictor of output isn't always perfect. 

The rough calculation is 50 watts per square foot. Seedlings and clones may require as low as 25 to 35 watts, while higher evaluations suggest up to 75 for flower phase. But, for simplicity's sake, stick with 50. 

Grow Tent Size Total Square Foot Total Watts Required
2 ft x 2 ft 4 sqft 200w
4 ft x 4 ft 16 sqft 500w
5 ft x 5 ft 25 sqft 1250w
8 ft x 8 ft 64 sqft 3200w 

While not perfect, wattage per square food adds another layer for your ongoing grow room light calculation

Useable PPF to Calculate LED Grow Light Requirements

First, a few definitions:

  • Photosynthetically Active Radiation (PAR), is the light spectrum plants use to photosynthesize, and therefore the only spectrum growers care about. 
  • Photosynthetic Photon Flux (PPF) is "the number of photons per second put out by a light source or how much PAR the fixture produces." 
  • A PAR map measures how much PPF is emitted per second by the light fixture, and most importantly — where that light falls. Directly under the fixture will always have more intense light than near the edges. 

So, let's look at the usable PPF and the PAR map for LED grow lights to help guide the decision-making process. Here are rough PPF requirements for cannabis at all stages:

  • Seedling: 200 to 400 μmole/s
  • Vegetative: 300 to 600 μmole/s
  • Flower: 800 to 1000 μmole/s

In your LED grow light research, ensure to examine the manufacturer provided PAR map. These are provided by hang-height, so take this into consideration as well.

With the PAR map in hand, plan accordingly. For example, for a flower room, does the PPF reading hit at least 800 μmole/s across the entire map or just across a small centralized area? If the spread isn't wide enough for your canopy, you'll need to add more grow lights.

Simple Solutions to Technical Questions 

Are grow tent size, wattage, PAR, and usable PPF all the factors in figuring out the LED grow light size? Of course not. 

How intensely you prune your plants, what training technique you use, how high you hang the light, and veg vs flower are additional factors. The deeper you go into the world of grow lights, the more complicated it can feel. 

The good news? Even if you stick with the most straightforward strategy— fitting a grow light size to the grow tent size —you will likely produce an impressive harvest. Keep it simple for the best results, especially in the beginning. Growing cannabis indoors is about constant evolution and experimentation. With a few harvests under your belt, you can add lights, upgrade fixtures, and improve your yields gram by gram.

Author | Chris McDonald

With two decades of expertise, Chris leads Happy Hydro in redefining sustainable gardening and delights in backpacking adventures, mind-expanding journeys, and creating memories with his loved ones.

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