What You Need To Know When Buying a LED Grow Light

Marijuana is a light intensive crop. As it’s maturing, it can take high light intensity. If weed plants do not get the amount of light that they require, then they will stretch towards the light source and also produce less yield.

The buds that will eventually be harvested are the plants’ flowers and to produce a lot of flowers, plants’ photosynthesis needs to be fully stimulated. This is achieved by giving the plants the right nutrients, good grow medium (soil, coco, hydro), the right pH water, and, arguably the most important factor:  LIGHT

Can a $50 LED grow light give enough light output and coverage to grow several large cannabis plants?
Do I need an expensive, let’s say $500, grow light to grow a single plant?
Also no.

So what are the deciding factors?
Your grow area
Adequate coverage and light output
A balanced light spectrum
Efficiency (efficacy) of the lamp
Build quality of the lamp

But also, what factors are not important or what to look out for?

Draw power
Diode wattage
Diodes with lenses and/or reflector cups

1) Your Grow Area
First of all, you need to know roughly how large area your grow will occupy. When growing indoors an enclosed area is recommended to control temperature and humidity easily. A grow tent, closet, or a small room are ideal locations. Grow tents have mylar walls which reflect light well. A closet or a small room should have white walls or be fitted with mylar to make an ideal grow space where as much as possible of the light is reflected onto the plant. The closer the walls are to the plant, the less light is wasted.

In terms of space or area, experienced growers will know roughly how large their plants will grow and can easily calculate how large are they will occupy. For beginner growers, it’s a bit of a challenge. The area of a plant has a lot to do with the plant’s pot size. The smaller the pot, the less room the roots have to grow and the smaller the plant will become. Other factors weigh in as well but that’s for a different discussion.
In short: one single cannabis plant in a 3 gallon (~10l) pot will occupy roughly 1.0×1.0’ to 1.5×1.5’ (30×30 to 45×45 cm) if it’s allowed to grow without much management. With a bit of LST (Low Stress Training), SCROG (Screen of Green) and defoliating, the plant’s size is better managed and can be formed to one’s needs.

With this rule of thumb you can calculate how large area you project will require. After that we can determine what lamp(s) will be a good fit for the required area.

2) Light Output and Coverage
As we briefly talked about in the beginning, cannabis plants will need light to produce flowers (buds). If the plants do not get adequate amount of light, their bud production will not reach full potential. In other words, to get the biggest possible yield per plant, a lamp that produces the required amount of light is needed.

Generally speaking this rule of thumb applies: the higher the cost of a LED grow light, the more light it puts out.

Efficiency (efficacy), light quality (spectrum), and build quality also matters but we’re going to keep things simple for now.

With this in mind, it’s not a great idea to go cheap on a light to save a few bucks. The money you save on a cheap light will result in a smaller yield. In other words, spending the right amount of a lamp will pay of when it’s time to harvest. Still, if you’re on a budget or primarily growing as a hobby and don’t care too much about cost:return then a cheap lamp might still be the right lamp for you.

The size of a grow light also dictates its price. A large lamp costs more to manufacture than a small lamp. Make sure to check the dimensions of the lamps that you’re considering as the size of the lamp is very much related to how large area the lamp will illuminate. 

Growing only a single plant will not require a large area to be illuminated but if you’re growing several plants, the light coverage, or the light footprint becomes increasingly more important.

Manufacturers should give you spec sheets or data that shows their lamps’ light footprints so that you can determine which lamp gives enough light for your grow area. Here’s how to read light footprint maps.

Light intensity, or rather, amount/quantity of light is measured in photons. All light consists of photons. The more photons we have per area, the more intense (higher quantity) is the light.
The term we use to measure the amount of photons per area is PPFD, Photosynthetic Photon Flux Density. We measure them in the unit umol/m2/s which means micromoles per square meter per second. I.e. how many photons hit a specific area every second.
Light intensity is also sometimes described as “PAR” instead of PPFD but the theory is the same.

Cannabis plants will require different amounts of light depending on their grow phase. When the plants are small seedlings, they need much less light than when they are mature. 

PPFD requirements for cannabis plants depending on stage:
Seedlings/clones: 100-200 PPFD
Vegetative stage: 200-500 PPFD
Flowering/bloom stage: 500-700 PPFD

If all other aspects like nutrients, temperature, and humidity are ideal and also adding CO2, weed plants can take a bit higher PPFD but the above numbers are good starting points for beginners.
The entire plant doesn’t have to be illuminated with these exact numbers but the majority of the grow area should be at least close to these PPFD levels.
Now that we know what PPFD levels to aim for, let’s take a look at a couple of real world examples where manufacturers present their lamps’ specs.


3) Balanced Light Spectrum
Plants have been growing under natural sunlight ever since the first plants came about. Natural sunlight consists of the entire light spectrum ranging from ultraviolet and blue all the way up to red and far red. In between there’s green, yellow, and orange. 

With LED grow lights their light spectrum can be customized. This means that a LED grow light can have specific colors. Each diode can emit a certain wavelength (color). A growing trend that pro growers avoid but beginners easily fall for is the change from a white color spectrum to a “BLURPLE”. A spectrum that is heavy on or consists exclusively of blue and red diodes. This trend was started only a few years ago by Chinese manufacturers who tried to cut costs and corners to make their light cheaper and as they argued, better. The false idea behind this trend is that plants do not need green, yellow and orange light so why waste money putting in diodes or a spectrum with these colors.

It is true that plants utilize blue and red light more than other colors for growth and mass production but the other colors also have their benefits. While a natural white light may not be the absolutely most efficient spectrum for plant growth, a spectrum that only consists of blue and red is far less beneficial. A balanced spectrum but possible with a blue and red boost is generally considered the best for maximum harvest and healthy plants. Experts say that somewhere in the neighbourhood of just below 20% blue light, about the same amount of green (~20%) and remaining red (~60%, or a little higher) is a good choice to boost photosynthesis and flowering. This is a good spectrum for the entire grow cycle.

During a plant’s vegetative stage it prefers a more blue light but during flowering it prefers more red light. Don’t pay too much attention to this during your first grow but once you’ve mastered the basic growing strategies you may want to add a bit extra blue light during veg and a bit extra red during flowering (perhaps by hanging a supplemental light bulb with these colors).

In addition, the “temperature” or shade/tone of white light is measured in Kelvin (K). If a grow light only uses white diodes then a K rating will be presented. A good Kelvin for the entire grow cycle is 3500K. A lower rating, let’s say 2000K, is heavier on red whereas a higher Kelvin, let’s say 5000K is heavier on blue. 

Kelvin rating only applies to pure white light and not to LED light that has a mix of various colored diodes as this would emit a light color that is not purely white.

In short: choose a grow lamp that produces either balanced spectrum with blue, green, (yellow,) red — not just blue and red. Or, if the lamps you’re looking at have white diodes, go for 3500K if the lamp is intended to provide light for an entire plant’s life cycle.

4) Lamp Efficiency (efficacy)
To produce photons grow lights converts electricity through drivers and diodes into light — photons. It’s the photons that are actually utilized by the plants throughout their photosynthesis chemical reactions. Not all LED lamps are equally good or efficient at turning electricity into photons. The measurement or term that describes how well a lamp does this is efficacy. Efficacy is measured in umol/J (micromoles per Joule) or PPF/W. The higher the efficacy rating, the more efficient is the lamp. 

Poorly built LED grow lamps, or lamps with cheap parts have low efficacy of around 0.7 umol/J. I haven’t seen lamps that are lower than this at least. Average LED grow lights within the beginner/budget segment typically run at around 1.0 umol/J and high end grow lights, COBs and QBs clock in between 1.5 to 2.5 umol/J. Generally speaking, the higher the efficacy, the more expensive is the lamp. Quality parts cost money which then translates to a higher sales price.

While efficacy is important, it may not mean all that much without seeing the bigger picture. So how should the efficacy rating be interpreted? 

We know that efficacy is a measurement of how well a lamp produces photons (light) out of electricity so…

Example 1: A lamp that is rated at 0.7 umol/J and draws 100W of power would only produce half as many photons (light) as another lamp that is rated at 1.4 umol/J and also draws the same, 100W, of power.
I.e. if the draw power for Lamp A and Lamp B is the same but Lamp A has twice the efficacy of Lamp B, Lamp A will emit twice as much light.

Example 2: Lamp C and Lamp D both produce the same amount of photons (light) but Lamp C has 0.7 umol/J whereas Lamp D has 1.4 umol/J, then Lamp D will only need to draw half as much power.
I.e. if the total light output for two lamps (C & D) is the same, but Lamp C draws 100W whereas Lamp D draws 50W then Lamp D has twice has high efficacy as Lamp C.

What this also means is that the total draw power (consumed watts) is not an accurate way of comparing two lights, as many beginners believe. As we see in the example above, two lights can have vastly different draw power while still producing the same amount of light. Efficacy matters!

5) Build Quality
As with most products, items, and goods out there: you get what you pay for. Blindly following the herd or buying the cheapest item on the market is rarely a recipe for success. Especially when the goal of any indoor weed grow is a plentiful harvest. Be critical. Be thorough. Ask questions. Contact manufacturers. Grill them, quiz them. Ask them difficult questions and see if they know the products that they are selling. Many won’t be able to answer in depth questions or sometimes even simple questions. Apply what you’ve learnt so far. Does the manufacturer provide a light footprint map? Is their PPFD/PAR chart accurate? How does the lamp look inside its housing? Is it well constructed or are there loose wires and loose parts?

A responsive company that is transparent and knows their product also likely has a good product that they stand behind. 

Online marketplaces like Amazon and Ebay are getting flooded with Chinese sellers. Although Chinese brands and products from China aren’t necessarily bad, we’ve all heard not-so-pleasant stories. Afterall, a country where marijuana is very much forbidden and consuming even in the smallest amounts leads to incredibly harsh punishment, a country like that cannot foster grow light brands that truly test their products. I find it hard to truly trust a company that can’t even test their products in their natural environment: in a grow tent, growing weed. Tomatoes and other vegetables may work as a somewhat close substitute but it will never be the same. Would you trust a sports brand that doesn’t support athletes or sports in general?

We also see Chinese brands come and go. They compete mainly by price and often by cutting costs on parts that are necessary. You can only push prices down so much before the product gets compromised. It’s difficult to “look under the hood” of a grow light and even more difficult to compare the internal parts between two lights. When a product appears to be too cheap, you should be skeptical. I sure am.

Look for: non generic lamps. If you see the same basic, generic design and build used by a vast number of brands, especially when they are budget brands, it usually means that it’s the cheapest possible build/design on the market — not the best.

See if the manufacturer is talking about the lamp’s internal parts or provides detailed specifications. Do they have any video based tests? On Youtube or on their website? 

Lastly. Beware of online reviews. Amazon and Ebay are losing their authority day by day as they get flooded with fake and bought reviews. Even articles with thousands of reviews shouldn’t be trusted just like that. Be critical and gather information of a product from more than one source.

6) Draw Power is Misleading!
I talked about this a bit in a previous point when discussing efficacy. Bulb based light technologies (HIDs) like HPS, MH, CMH are easily comparable by their draw power — how many watts they consume. There’s little difference from bulb to bulb and these fixtures are similar in terms of efficacy as well (at least within the same wattage). LED lights, not so much.

Just because a LED light fixture has high draw power doesn’t mean it’ll put out a lot of light. Once again, this is a battery of efficacy; how well a lamp converts electricity into light.

A lamp with low efficacy could consume a whole lot of watts but still not produce as much light as another lamp with low draw power but high efficacy.

Always take efficacy in the unit umol/J into account when looking at two lamps’ draw powers.

7) Diode Wattage is Even More Misleading!
Many brands in the budget segment (this is less of an issue with high end lights, $500>) are marketing their products based on the total diode wattage.
Let’s say a LED grow light has 100 diodes, each of them with a theoretical maximum capacity of 10W. This light would be marketed as a “1000W” lamp as 100 diodes times 10W equal 1000W.

While 1000W sounds incredibly strong and powerful, this doesn’t tell a single thing about a lamp’s actual light output. 

One of these lights could be drawing 160W. That means each diode is only actually consuming 1.6W (160W divided by 100 diodes = 1.6W). This is what matters, combined with efficacy.
A diodes “maximum theoretical wattage”, 10W in this case, means nothing if the diode is only running at 1.6W. A 2W diode would be just as good in this case. Same goes for all these “double ended” or “triple ended” diodes. If their watt consumption is low then they could be “hundred ended” and still barely produce any light. 

So. Look for the watts consumed and efficacy. Not any inflated wattage number that doesn’t provide any indication of a lamp’s performance or output.

8) Diodes With Lenses and/or Reflector Cups Focus Light Can Be Misleading!
When browsing LED grow lights there will be many products that talk about focusing lenses and reflector cups. Both of these alternatives can be good or bad, all depending on your grow setup.

Reflectors and lenses alike have the same purpose. They are to narrow the diodes beam angle.
A diode without a lens or reflector would shine at a 180 degree angle. This means that the spread of light is quite significant. If the goal is to illuminate a large area, a wide beam angle is a good thing. However, if the light is only supposed to illuminate a small grow area of a single plant, then a narrow beam angle is better. 

The negative thing about lenses and reflectors is that some light (photons) is wasted when passing through or when being reflected. Depending on material and construction it can be anything from one to ten percent. Loss is never ideal as each emitted photon is created by consumed electricity, electricity that you pay for. 

However, if the lens or reflector makes the majority of the emitted light land where you want it to, i.e. on your plants instead of walls or on the floor, then it’s a good thing.

Some brands use this trick to boast with inflated PPFD/PAR values. Since focusing light will create more intense hotspots (area where the light is focused onto), some PPFD/PAR values will be very high which creates a false sense of high performance. If the light footprint of this light is analyzed and not just a single spot measurement, the footprint will likely show that due to focusing the light, the footprint will be very narrow. Light won’t reach very far from the center spot. This often leads to burnt leaves or plants on certain points whereas other parts of the plant are suffering from light deficiency. 

Consider the light footprint instead of single, centerspot, measurements to get a fair understanding of how well your specific grow area is covered with light. 

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