Footprints and Watts
When purchasing any grow light, you need to consider how much light (power) will be delivered to the area you want to grow in, and LED grow lights are no different. There are a lot of things to consider, as I detail below. In the table above I've ranked companies based on the likelihood of having enough lighting power to assuredly flower Cannabis within all of the lights' smallest advertised footprint. If every flowering or general-purpose light model offered by a company has enough effective watts per square foot to flower Cannabis, they went in "The Good" column. Companies which have at least one model that has enough effective watts per square foot to possibly flower Cannabis (or who don't provide enough information to know) went in "The Questionable" column. The companies in "The Bad" column don't offer a single light model which I feel could flower Cannabis within all of their smallest advertised footprint.
What do They Mean by That?
Most vendors provide a claimed coverage area for their lights and some give different footprints areas with different names, which could make sense as different types of plants have different lighting intensity requirements, some even for different stages of their lifecycle. It would be impossible for an LED seller to give appropriate footprints for every kind of plant.
So it kind of makes sense that different sellers are giving different names to their advertised footprint(s), but this can be frustrating because you don't necessarily know what they really mean.
Some vendors only give a single "coverage area" or "footprint". It isn't clear whether this means you could grow low-light plants, high-light plants, or flower cannabis within that footprint. Hopefully it means you can grow and flower high-light, daylength-sensitive plants such as Cannabis in that area, but I guess the only real way to find out would be to try it yourself.
I've always assumed that if a seller is giving different "flowering" and "vegetative" footprints, they probably mean it to apply to Cannabis, but my research has me thinking that's not a safe assumption. By "flowering" they could be talking about flowering African violets, Cannabis, or barrel cactus, and there's a steady increase in the respective light requirements for flowering those plants. The only LED grow light company that I could find with an explicit statement of what they mean by "vegetative" and "flowering" footprints is Black Dog LED; they claim the footprints given apply to Cannabis. It would be nice if the other sellers were explicit about what kind of plant they mean, but this problem isn't limited to LEDs; HPS and other grow lights are most often sold with ambiguous footprint specifications as well.
Several sellers give two footprint sizes labeled "total coverage" and "core coverage" or some slight variation on that theme. This does kind of make sense; with LED lights, the individual LEDs each illuminate a circular area directly underneath them. Where these circles all overlap, the light is most intense, and this could be called "core coverage". Toward the edges of the footprint, less of the circles overlap and this could be interpreted as "total coverage". So for this total / core theme, the two footprints are probably achieved with the same light hanging height, whereas the veg / flowering footprints usually involve raising the light up a bit to get the larger veg footprint.
Still, "core coverage" doesn't necessarily mean that you could use it for high-light plants or for flowering Cannabis; you're left to make assumptions about what it really means.
Sometimes the names are so cryptic that it isn't clear what the seller means at all. Here are some notable examples:
- For the Sol series of lights, two footprints are given: "Supplemental Coverage" and "Primary Coverage". While this seems like it would be a variation on the "total"/"core" theme, a little quick trigonometry to figure the hanging heights required to achieve each of these footprints given any reasonable estimate of the light spread angle (from the picture of the lights' reflector) shows this is almost certainly not the case. With power densities down to 0.78 watts per square foot, I'm pretty certain the definition of "supplemental coverage" might not include "keeping plants alive" let alone getting them to grow. But, the word "supplemental" already implies that you would need other lighting to grow plants in the footprint.
- For the H350 light, they give two different-size footprints (4 and 9 square feet), both called "coverage area". This really isn't helpful.
Watts per Square Foot
One of the fairer ways of comparing the efficiency of all the different HID, fluorescent (/induction), plasma and LED lighting options is to look at the number of watts of power they require per square foot of plant-growing footprint. Since the varying footprint names given by the sellers imply different things, it is hard to make a fair across-the-board comparison. Of the lights I review here, the watts per square foot (W/ft2) range from 0.78 W/ft2 for "supplemental coverage" all the way up to 48.3 W/ft2 for "Intense Coverage"– more than a 60X difference, but because the footprint names are vague I can't really be sure what the seller meant.
So how can you tell how many watts per square foot you really need? It's going to depend on 3 things:
- The light intensity you need to grow or flower your plants. This depends mostly on what you are trying to grow and/or flower.
- How efficient the LED grow light is at converting a watt into light that makes it to your plants. Some LED lights are going to be more efficient than others, and new technology is always coming out, so this isn't necessarily going to be consistent between different LED lights.
- How efficient the light's spectrum is at growing and/or flowering plants. Since so many different spectrums are being offered, this isn't going to consistent between different LED grow lights either.
With all these considerations, I can't tell you for sure how many watts per square foot you will need for whatever you want to grow, but you may be able to figure out a guideline range yourself.
Since most people using artificial lighting are probably growing Cannabis, I will attempt to provide some guidance for flowering Cannabis. In the world of HPS lighting, most people seem to agree that you need at least 50 watts per square foot to get good results (a 1000W HPS over about a 4.5'x4.5' area, or a 600W HPS over about a 3.5'x3.5' area), but you're going to get better results with 63-67 watts per square foot (a 1000W HPS over a 4'x4' area, or a 600W HPS over a 3'x3' area). Many people use even more watts of HPS per square foot than that.
So I'm going to call the recommended range for HPS 50-67 watts per square foot. If an LED light is 50% more efficient than HPS lights at growing plants, which I believe is a fair upper-end assumption based on what I've seen so far, that means you need 1/3 less watts per square foot, a range of 32-45 watts per square foot. The table below shows W/ft2 ranges for different assumptions about how much more efficient LED grow lights can be compared to HPS:
|Assumed LED efficiency increase over HPS||LED W/ft2 to equate to HPS at:||My opinion about whether is is realistic to flower Cannabis|
|50 W/ft2||67 W/ft2|
|100% (double the efficiency)||25||34||Unlikely|
|200% (triple the efficiency)||17||22||No|
The column in the spreadsheet "Likelihood of sufficient power to flower Cannabis within smallest advertised footprint if on a light mover" looks at the footprint area(s) specified by the seller (it takes the smallest footprint with the highest watts per square foot if there are two footprints) and labels it with my opinions according to the above table for the equivalent to 50 W/ft2 for HPS. This column represents my opinion only about the power density for the advertised footprint square feet; it is important to keep in mind all of the following:
- If the spectrum is poor, power density isn't enough to flower plants well.
- If the light distribution is uneven, you may get spotty results due to:
- secondary lenses
- a poorly-shaped advertised footprint— I got burned by this myself, so I added a better comparison this year that accounts for the footprint shape. If you put the light on a light mover and only cover the advertised footprint, it eliminates this concern.
- If they give 2 footprints remember that this label only applies to the smaller of the two.
- Of course, you can re-analyze the numbers using whatever thresholds you would like!
Of the companies that actually give a "flowering" or "bloom" footprint, only Area 51, Black Dog LED and California Light Works have more than my recommended minimum 32 W/ft2 for all their lights. All of Lush's general-purpose lights qualify as well with the more generic footprint name “Intense Coverage”, but only if you don't consider footprint shape.
Note that this is all about comparing watts per square foot for the entire advertised footprint; if a light is ranked poorly it does not mean it will not flower Cannabis plants at all, just that it is less likely to be able to flower Cannabis within the entire area of the advertised footprint. To achieve better results, You can get a light and move it closer to the plants to shrink the footprint area and increase the W/ft2, or just put multiple lights over the same footprint area.
For example, if you put 7 of the Pro Max Grow MAX-600 models over the advertised 4'x4' footprint for a total of 36 W/ft2, I'm fairly certain the setup would flower Cannabis well. Likewise, if you just moved the MAX-600 closer to the plants and decreased the footprint size to 3.25 square feet by making it roughly 27" by 17", you would get 32 W/ft2, and that should work well too. But putting 1 MAX-600 above a 4'x4' growing area as advertised with 5.15 W/ft2 will not flower Cannabis over the entire area.
Effective Watts per Square Foot
In 2012 when I first put up this site, one of my main qualifications was watts per square foot. I labeled several lights as "Top Picks" based on the results of my analysis, and then bought one of these I had not tried before. It had enough watts per square foot that it should flower well, but it didn't flower well at all for me using the seller's recommended footprint. The results were inconsistent, with some plants doing OK but others obviously not getting enough light.
It took me a while to figure out the problem was that the seller had specified an inappropriate shape for their footprint–if I kept the same total area but changed the shape to match how the light was actually spreading out, the light did a better job of flowering.
This is why I've added the discussion and calculations of footprint shape to the site now. Basically, if the advertised footprint shape is wrong but you use that as your growing setup, you're wasting either growing area in the footprint or light off the ends (or both). I've come up with a percentage coverage score that represents how much area or light is actually usable, based on the lights' shapes and the way light spreads out from LEDs.
By multiplying the watts per square foot for a footprint by the coverage score, we get the true "effective watts per square foot" for the advertised lighting footprint; I've added this column to the spreadsheet for the smallest advertised footprint area, and the column "Likelihood of sufficient power to flower Cannabis within smallest advertised footprint" uses the scheme I set out above to show the probability of the light flowering Cannabis in the entire smallest advertised footprint when you account for the footprint shape.
Since I can't really make adjustments for spectrum or secondary lenses that may cause spotty coverage, this is the most objective score I was able to come up with for the likelihood of the light flowering Cannabis in the seller's advertised footprint.
If the seller didn't provide information on their light's shape, recommended footprint, or actual wattage I can't calculate this score; in these cases the column just shows "?".
To summarize again, I feel anything greater than 32 in the graph below is very likely to flower Cannabis well; 29-32 is possible, 25-29 is unlikely, 20-25 is highly unlikely, and anything below 20 will not flower Cannabis well at all.