LED lights for home use have become very popular and many people are considering them for growing plants, especially for starting seedlings. It sounds like a great idea. LED lights use very little electricity so they save money, and they don’t produce a lot heat so they can be placed close to your plants. Sounds perfect and many people have started to use them with success. Unfortunately, others have found that they don’t work too well and one reason is that manufacturers don’t tell you the truth about their products. I know that is hard to believe – but it’s true, they go out of their way to confuse you.
In this post I will have a close look at just one aspect of LED grow lights – the truth about watts.
LED Grow Lights – What is a Watt?
A watt is a unit of measure that describes the power used by an electrical circuit. Historically, consumers have used watts as a way to measure the amount of light produced by a light bulb. A 100 watt light bulb is much brighter than a 60 watt light bulb and two different brands of 100 watt incandescent bulbs have about the same brightness, because they use mostly the same technology.
If watts are an indication of brightness, then we should be able to use it as a way of comparing different types of LED grow lights and some manufacturers agree. They will try to sell you on the watts. Brand “A” at 20 watts is better than brand “B” at only 15 watts.
Unfortunately, it does not work that way for LED. A 100 watt light might actually have less useful light for plants than a 50 watt light. Here are some reasons why.
LED Bulbs Are Not Equal
LED technology is moving ahead very fast and not all LED bulbs are of equal quality. You can have two 3 watt LED bulbs that produce very different amounts of light.
LED lights need electronic circuits to run them. The quality of the circuits vary a lot from one manufacturer to another. As a consumer you have no real way to understand your options, and in most cases the manufacturer is not telling you what they do – even if you could understand it.
Unlike traditional incandescent bulbs, the LED technology varies a lot.
Efficiency
The attraction of LED lights is that a greater amount of electrical energy is converted to light photons – they are more efficient than older technology.
Within LED lights, there is also a great variation of efficiency. Consumers have been lead to think that almost 100% of the electricity is converted to light, but that is not true. A good LED is about 80% efficient. The extra 20% is converted to heat.
Efficiency also depends very much on the quality of the LED and this can vary by 200% between manufacturers. There are good reasons why some LED grow lights are cheap and others are expensive. To some degree, you get what you pay for.
A higher efficiency, will produce more light per watt, and cost less to run, relative to the amount of light produced. The problem is that consumers has no way to know the efficiency of a particular brand of grow light.
% Capacity Matters
The LED bulbs come in 1, 3, 5 and 10 watt models. If you were comparing two fixtures, one with 100 x 1 watt bulbs and the other with 100 x 5 watt bulbs, you would reasonable conclude that the latter produces five times as much light (assuming equal efficiency). You would be wrong. The 100 x 5 watt unit probably has circuits that run at about 50% capacity. That means the 100 x 5 watt unit is actually only 2.5 times as bright.
The reason manufacturers do this is because higher watt bulbs generate much more heat and this heat shortens the life of the bulb. It is common to run high watt bulbs at 40% or even lower. Therefore they don’t produce the light you would expect based on the watt rating.
The designations, 1 watt, 3 watt and 5 watt are still used a lot, but the newer technology is muddying the water. A new high-end LED, the Cree XLamp XP-L, can be used at any wattage between 1 and 10 , depending on the circuit.
This video is from a manufacturer, so you need to take the information with a bit skepticism, but it does show how the lamp watts are not the only thing that matters.
If the above video does not play, try this link: https://www.youtube.com/watch?v=RFYLQGxux5U
Lens Design is Important
The lens is the plastic cover over the actual diode that produces the light. A lens can focus the light or spread it out.
A manufacturer may claim to cover a wider area, but that usually means less light for any given spot. It is however attractive as a selling point. Why would you not want a larger area covered for the same price? A light that is spread out means less light for any given leaf, which usually results in slower growth.
There is currently no single metric that allows you to compare the lens of one manufacturer to another.
Fans Also Use Watts
Fixtures with more bulbs in them run hotter. Higher watt bulbs also run hotter. If the bulbs gets too hot, they burn out so manufacturers making higher watt lights cool them with fans. That is not really a problem. Compared to older technology, LED lights still run cooler.
But there is a problem. Fans also use electricity – they use watts. When a manufacturer reports that they have a 250 watt light, the 250 includes the watts used by the fans.
So does a 400 watt light fixture produce more light than a 250 watt fixture? Maybe, or maybe not. The 400 watt unit may use more fans and produce the same amount of light.
Watts Per Surface Area
Most consumers equate watts to light output and so they go shopping for watts. But how many watts do you need? There are tables that provide recommended watts per unit area so if you wanted to provide light to seedlings in an area of 8 sq ft (2 ft x 4 ft), you could look up the total watts you need.
If you followed my comments above, you now realize that any such table is very approximate, but there are even more factors that make such tables almost useless.
The height of the fixture plays a big role in determining how much light any given area receives. If you move the light from a height of 1 ft to 2 ft, you will reduce the light by 75%, according to the inverse square rule, which applies to a single point of light. At a height of 2 feet it covers a bigger area, but the light is so much weaker.
The light pictured above is not a single point light and it shows a 50% reduction when the distance is doubled. You can also see how dramatically the light level drops as you move away from the center of the light.
The watts required also depend on the plants you are growing. Seedlings need low levels of light. Blooming African violets need more, and blooming orchids much more.
The manufacturers website for the light shown above recommends a height of 2 feet where it will cover a width of 3.8 ft. The same light on Amazon also recommends a height of 2 feet, but it covers 5.1 ft. How can the exact same light perform differently depending on where you buy it?
I know, even after reading the above you still want a rule, so here it is. In very general terms you need 15 watts per sq foot to grow seedlings.
Wattage Means Very Little
Watts is a measure of electricity used, not of the amount of light produced.
The conclusion here is that it is almost impossible to know how many watts you need. Although manufacturers try to convince you to shop based on watts, it is really a meaningless number. It is useless to use the number to compare grow lights. Very high watts usually produce more light than very low watt units, but that is not much use.
What number is important when buying an LED grow light from different manufacturers? You really want to know the amount of light and the wavelengths being produced .
If you go to sights like Amazon to buy your fixtures you won’t find these numbers. Consumers don’t ask for them, so sellers don’t provide them. For higher quality products you might find this information at the manufacturers website.
1 vs 3 vs 5 Watts
If all other things are equal, which size bulb is the best for growing plants? The 5 watt gives you the most light, but it also generates the most heat and the bulb is much more expensive than a 1 or 3.
With today’s technology the 3 watt bulb is the best choice. It is a good balance between light generated, heat produced and cost.
LED Grow Lights vs Florescent T5 Shop Lights
The following video compares some inexpensive LED lights with traditional T5 florescent lights. You will notice that the author makes a big deal about the watts used by each fixture, but the difference in values does not mean very much.
Having said that, the actual values should at least match the advertised values and so this video demonstrates another problem with buying these lights – you can’t trust the numbers used to advertise the products. It is certainly a big problem with low cost LED grow light systems.
If the above does not play, try this link: https://www.youtube.com/watch?v=EtRWbltewB0
Unfortunately, the results in this video do not mean too much since only 1 plant was tested under each light. This kind of testing should have replicates.
What Does It All Mean?
Whatever the sales promotion tells you about watts – forget it. Watts tells you very little about how well a lamp will grow plants.
If you want to get technical and get all the right data, you are out of luck. Few manufacturers provide the data you need to compare different types of LED grow lights.
The industry is working on some standardized labeling, that will allow you to accurately compare two products, but this has not been adopted yet. The label below shows you what it might look like once adopted.
For most consumers, this label is far too complex. When adopted, consumers will still be confused but at least it will level the playing field. Right now manufacturers can say anything they want.
Thanks for sharing this information !
Amazing article…I found LED lighting is truly the next generation of technology which features various benefits to consumers.
Your information is WAY behind the curve. Yes, Chinese ebay merchants are still selling these ” burple” light, ie, RGB, but anyone who has made the mistake of buying one has moved on to much better technology, they have gone to COB lighting into their gardens. On that note also, you 80% efficiency is pie-in-the-sky. Real world best efficiency hovers around 62-65%, still double what HPS and those junk burple lights can meet. Only very expensive ” horticultural lighting can meet 55-60% efficiency using expensive LEDs.
Sure, you could get 80% efficiency but then you have light levels too low to make it worthwhile.
And using watts I agree, is a meaningless metric that tells you nothing about the light. Yet the metrics that do, people are too lazy to bother to learn and so, people continue to buy inefficient, poorly made, and sometimes UNSAFE lighting! I’ve seen the inside of some where the wires were twisted together, not soldered, tape instead of shrink tubing, and BARE drivers exposed to the grow environment! A fire waiting to happen!
So educate your a little better before posting an article like this, your about 4 year behind the curve!
1) Most gardeners buy on line, and they are buying the burple lights – they may not be the best, but that is what is commonly purchased.
2) LED can run at 80% efficiency. If you read the post it clearly says they don’t normally run at that level, and I have used the numbers 40 and 50% as being more representative.
So the post matches what is going on today and is not 4 years behind the times.
Don’t pay attention to self-righteous trolls. It was a great post. Very Helpful. Myself, call me old fashioned, but I don’t pay much attention to people who can’t spell or use coherent grammar.
Thanks so much for this timely article. I appreciate your ability to take a subject that just might make my head explode and put it into graspable context.
I have had good luck with my LED lights which are rated at 200 & 300 watts. I have a light meter which measures PAR which are the wavelengths that plants use for growth. This is a good way to test these lights. I found that the 200w LED lights produced much more useful light than my old 2 bulb 48” florescent fixture especially the in center and saved a lot of electricity. Light falloff can be an additional problem depending on design. I use 3 300 watt units in greenhouse for winter light for citrus and veggies which has worked quite well. I agree we need better labeling and for anything other than seedlings people need to think about exactly how to mount lights. great potential but you need to experiment to get it right. There are many high tech commercial greenhouses that rely exclusively on LED lights with great results. So they can work if you get the right ones and are careful about how you mount them.
Very thorough, but if I may add one more wrinkle, quoting from my new book THE EVER CURIOUS GARDENER, “Light from an LED spans a very narrow spectrum; if in red, just a narrow band in red, and similarly for blue or any other color. So narrow, in fact, that a different recipe for light seems to be needed for op- timum growth of different kinds of plants, or different stages of growth. Performance is generally enhanced with the addition of a small amount of light in the green as well as far-red spectrum (the part of the electromagnetic spectrum just beyond the red that we can see, but shorter wavelength than infra-red). LED “grow lights” are on the market, but research with plant growth under LEDs is in its infancy.”
Quite correct. Working on the next post about wavelengths.
As a physicist, I was flabbergasted at the existence of “white light LEDs” and knew that they were NOT Light Emitting Diodes, because such a diode emits radiation (photons) exactly and only at the energy level difference between the P-doped and N-doped regions of the di-ode!
It is therefore monochromatic.
Then I discovered that a white LED emits that light for exactly the same reason as a fluorescent tube. Its monochromatic radiation is in the ultraviolet (UV), and the white light is a combination of emissions of phosphors energised by the UV photons.
I’ve not seen an actual spectrum analysis of “white LED” light, but I’d expect it to be a few or several monochromatic “line” emissions. It would be no surprise if plants respond differently to what the human eye perceives as “the same” shade of white, from different manufacturers. Madder still, phosphors that emit in the green range are rather a waste for indoor plant lighting.
The reason that leaves are green is that chlorophyll can’t use that range of colour. It’s the colour of photosynthetic wasted radiation!
It is interesting that you say they are monochromatic – that is what I would have expected. The spectra of a single LED (not a white one) still shows a fairly wide wavelength band – I’ll show some examples in my next LED post. The light is not monochromatic at least not the light that reaches the plant.
I’ll also post some white light spectra in my next LED post. They are quite wide spectras, and nowhere near being several monochromatic lines.
It turns out that green light is actually used by plants and can be an important component of the light they get.
If Round-up affects plant by being absorbed by leaves, why does painting cut trunk or root surfaces damage weeds?
When painted on the cut trunk, glyphosate is absorbed through the phloem.
I think I will stick with my 10 year old fluorescent fixtures with a warm + cool bulb. Works great and the price is right.
As a matter of interest I used one of these grow lamps. It was fantastic. A huge difference….but, I used it in the winter to start seedlings off in January. One of the reasons it works in this situation is simply because I gave the plants 12 hours of light per day instead of the natural 6 or 8. Any extra light would have worked. Also, because I was growing in January I also provided heat, something I don’t do if starting tomatoes off in Jan normally. Whether the red/blue light helps that much compared to a standard lamp I don’t know because I didn’t test that but one of the claims the manufacturers say is that it produces stronger stems and causes more flowering, earlier. Both of these happened but whether that was because of the even number of hours (12 hrs per day) I gave it or whether that was because of the more stable and warmer conditions I don’t know or whether it was because of the red / blue lamp, no idea, but it worked, and worked well.
As for wattage, ignore it. Just put the lamp as close as possible, heat rises and doesn’t damage the plants. Even if the real brightness of the units differs, placed close enough gets rid of most of that problem if all you are trying to do is grow in the darker months rather than for a special reason.
As for coverage. The light from LEDs basically goes down so the lamp unit wants to cover the same area as the plants occupy but, and I have a photo of this somewhere, the plants under the lamp grew up – very well as expected, the plants even 3 or 5 feet away could still tell where the light was because all the seedlings from around the greenhouse were leaning toward the lamp unit even though they were no where near it. Light levels from the unit must have been 90% and less in some areas and the plants still responded. The problem wasn’t that they weren’t getting enough light but the fact they could see “some light” and it wasn’t coming from above but to the side, it made these seedlings leggy and fall over. Plants therefor will (in the right conditions) benefit from any extra light you give them when sunlight isn’t available.
The other thing with “how much light” or “wattage” is that you have no idea how much light normal day light produces compared to a lamp unit so can’t make an informed choice as to what wattage you want, but there is a way. Mobile phones often have a light meter in them, get an app to read the LUX light levels and take the phone outside make a note, then take the phone indoors or in the greenhouse or where ever you want your grow lamp (and even put it under your grow lamp) – massive difference. The sunlight produces a huge amount of light compared with one of these grow lamps. It’s amazing how much light the greenhouse blocks even with plain glass (with dirty windows).
Fascinating subject light is. Your living room light bulb provides very low level light by the time it hits the floor compared to sunlight hitting the floor.
Keep up the great posts
“you have no idea how much light normal day light produces” – we do know this number “the sun emits about 3.86 e26 watts of energy”.
The light meter in a phone is a good idea, but you need to buy the light to use it. Not much help in selecting a light.
the phone is useful if you’re in the shop and test the lamp 🙂
The comment of “you have no idea” wasn’t meant to sound like that 🙂 It was more about the difference in a light reading under a lamp and taking a reading outside, and comparing the greenhouse light level compared to outside – just a comment to say “it’s an amazing difference”.
Yes, “we” do know the real amount as a level of power but the general public have no idea and so can’t compare the power of a lamp with that of the sun to produce the required level of light that they may require.
The wattage numbers of the lamps are really only of use so you know how much electricity they will cost you – then they are very useful.
Didn’t mean to be disrespectful to you if that’s how you took it 🙂
Your comments were not disrespectful. Thanks for commenting.
Wow! Thank you for this !