LED Grow Lights - The Myth About Watts (2024)

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 – a topic for another post.

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.