A Discussion about Lighting Efficiency

I’ve been thinking about the efficiency of grow lights, and the factors that go into that efficiency. I’d like some input from those who are more versed in this than I am. Please correct my inevitable mistakes or misstatements and let’s discuss!

First, to define what I mean by efficiency in the context of growing cannabis. I think the easiest metric is watt-hours to yield in grams. Note, this is not grams per watt. Watt-hours are a measure of energy, where watts are a measure of power. Energy is what you pay for, because it is the cumulated power over time.

I realize that cannabis strains have a somewhat consistent flowering time within the strain. But that does vary, and so does the amount of veg time given to a plant. Is it fair to compare the yield of a plant vegged for 6 weeks with a plant vegged for 12 weeks under the same lights? The latter one uses twice the watt-hours. Plants vary within strains and between strains, so comparisons are of course a bit tenuous. But it’s the best metric for an efficiency rating that I can come up with. I’d like to hear others’ thoughts.

So the question is about the factors between the wall outlet(s) and the output of the plant. Let’s start with the light hardware itself.

The power drawn from the wall turns into a number of things in light fixtures. Light energy (photons), heat, and sound are three outputs from that energy. An electrical resistant heater emits a soft red glow (not a lot of light) but a lot of heat. LED lights emit more light and less heat. The measure of how much light is emitted per watt is called radiometric efficiency, and it varies wildly between different types of lights, and even between different subcategories. One source notes that LED radiometric efficiency can vary by as much as 200% between popular brands. This is why sizing your lighting based on watts from the wall is too far removed from actual light output, and a bad idea.

However, plants do not utilize visible light the same way our eyes do. PAR (photosynthetically active radiation) is the type of lighting wavelengths that are useful to the plant for photosynthesis. There are, as far as I can tell, three important measures of PAR:

  1. PPF: Photosynthetic Photon Flux. Measure of how much PAR is produced per second by the light.
  2. PPFD: PPF Density. Measure of how much PAR hits the plant at the light distance and angle. This is commonly measured at the middle of LED panels which is misleading.
  3. Photon Efficacy: The big one. How efficiently is the light turning watts into PAR.

So it seems to me that Photon Efficacy is as important as anything when it comes to comparing the energy efficiency of lights.

When I have a minute, I’d like to follow this up with another factor involved in efficiency: wasted light by missing plant surfaces, light mounted too high so that PPFD is lowered, or resulting in PPFD that’s higher than the plant can utilize, etc. I welcome responses and input.

@Bogleg @dbrn32 @MAXHeadRoom @MattyBear any anyone else who is interested.


Good topic! I’ll try to convey where I’m at on those points. Some may be more factual statements than others which may be my thoughts based on what I’ve seen with my own eyes, and could be subjective to certain variables. Some of which may or may not have been into play.

When you talk about photosynthetic photon flux, photosynthetic photon flux density, and photon efficacy, the first two talk about light levels and last how good your light is. Why can some leds produce like a 1000 watt hid at less power, yet others seem to be advertised as such and are nothing close? That’s you’re photon efficacy, usually expressed in umols per joule. How much total radiometric energy your light emits (notice I said total not what your canopy sees) is the ppf usually expressed in umols per second. And finally the amount of ppf over a given area is your ppfd, usually expressed as umols per second per meter squared.

So technically speaking your ppfd doesn’t really change as your distance to canopy changes, but how it’s measured does. You still have the same amount of photons in the same amount space. It’s what happens to those photons over the distance that changes. The further your light gets away, the more they disperse where they’re free to bounce around. It’s the same, but different. You take an amazon light that shows a spot measurement of 1500 umols directly underneath it at 18” and compare that to a 6 cob light from timber with advertised ppfd of 1000 umols, and think the amazon light is better, you’ll be very upset. It sucks, but is what it is. You simply just can’t compare a spot measurement with overall light output. But the amazon light probably does measure as such at that point, but falls flat on its face with minimal movements from center.

I like the way you’re looking at cost of lighting over the entire grow, including veg time. A plant vegged weeks won’t produce as much as a plant vegged 12 weeks. But how much area does each plant take up? Is it possible that 4 plants on 6 weeks veg time could fit in the same area in the grow space as the single plant vegged 12 weeks? If so, what’s the potential that those 4 plants would out produce the single plant vegged 12 weeks? We already know 6 weeks veg time will be cheaper on the bill than 12 weeks, but what does it take to create the same amount of canopy? That’s going to vary a lot from grow to grow, but it’s nots a new thought or practice trading plant count for veg time. Which ultimately pushes the ideal measure here from grams per watt, to grams per kWh as you suggested, to grams per kwh per square foot.

No intention of sidetracking from what you’re doing, because I like it a lot! But there’s so much more to the bottom line than lights. But the principal is there. First thing is to meet required light levels, after that should be to do with the most efficient light one can afford. Measuring watt per square foot is relatively useless outside of the cheaper led fixtures. Since most of them are the same though, a few blanket statements can be made. But definitely doesn’t apply to every light just because it’s an led.


I agree with your criteria entirely: Watt-hours to yield in grams is exactly what matters. As far as efficiency, the best COBs put out about 200 lumens per watt but you have to run them at about 1/4 max power to get that. So you have to buy 4 times as many COBs as you initially thought! But even at that super efficient operation point, the best COBs use about half of their power to warm the heat sink. Out of the half of the watts that becomes light, only about 6% gets turned into sugar by the leaves (agricultural efficiency of cannabis), and that is the light that hits a leaf. Light that never hits leaves just gets turned into heat by the tent walls.

The one thing that I know you can do to increase efficiency easily, is to use hydro. I’ve used both coco coir (which is hydro) and potting soil, and the veg time in coco is very much less. I assume other hydro mediums are the same.

Another thing to think about is spectrum. The conventional wisdom is MH for vegetation, HPS for flowering. When you use LEDs, you get a lot more choices. In particular, COBs come in different color temperatures. I use 5000 K for veg and I’m adding some 3500 K and some 640 nm and 660 nm LEDs for flowering. PAR meters don’t do a very good job telling you anything about your spectrum, but I suspect spectrum matters to the plants. PAR meters assume your greenhouse is sun lit, and then let you compare one greenhouse to another. When you use artificial light the PAR meter might say you are in good shape but your spectrum sucks.


I agree completely with assessment. What I find astonishing as far as dealing with LED is that they are far too powerful for sprouts unless individuals are striving to attain smaller home counter size plants. To me this indicates that PAR is a huge indicator of exactly how the plant processes it’s growth. I also think that the majority of growers have underestimated exactly what tLED are. Current technology has propelled us much further in achieving plant growth parameters. Factors like flower veg. And stretch all come from sunlight combined with genetics what we can do is control height strength width and both veg. And flower times. I use only LED and i would love a comparison of LED/CREE compared to ballast form. I am running tent plus tv/entertainment system in room constantly. I can produce 1 pound of weed every 5 months, effortlessly. My bill lowered the very first day I connected what was an experiment. Hell I’m trying to figure out a way the plant itself will give me more power! I’m just saying production cost to energy usage is perfectly logical. Sorry an “I got so stoned moment”


If you’ll trust YouTube results, you can find quite a few side by side grows cob vs hps. Most of the guys doing it have now switched to cree, citizen, bridgelux, Samsung type of side by side grows if that tells you anything.


It’s really an initial cost versus operating cost tradeoff. If you are only in it for a year or two, go HID. If you want long term performance, go COBs. HID kits are cheap, from competition with LEDs. Magnetic ballasts should last for years but bulbs need replacing every year or two. LEDs or COBs should last for many years if they are run at conservative currents with good heat sinking, but cost more upfront. HID probably cost a little more to run, but not much more. Like 35% more or less, depending on how good the COBs or LEDs and the driver is. It is certainly possible to find a lousy LED lamp that is less efficient than HPS.


Thanks for the responses. Interesting stuff.

@dbrn32, thanks for the clarity on the metrics, especially the PPFD. Also, the concept of more plants with less veg time. The less time light is hitting the tent, the more efficiency. I bet that’ll even outweigh some of the fixture efficiencies when tweaked properly.

@1BigFella, you’re exactly where I was going with this. How does the spectrum matter, and the intensity at different wavelengths within the spectrum? A light might have an ideal spectrum, but if it peaks in areas where the plant isn’t as capable of metabolizing the light, that’s a loss of efficiency. The remarks about hydro/coco vs. soil are really valuable. If one can shorten veg time with the same results, that’s a big bump in efficiency. Do we have any data on the difference? Seems to me that’s a great way to minimize seed to harvest.

@Viktor7, I’ve seen the same and had that same issue even with my Amazon panel LED. My plants became very stout even though I wanted some stretch to reach my SCROG. I had to bump the plants up significantly. And in the process of waiting, I was burning valuable kWh, much of them falling on empty space.

My motivation here isn’t just to save a few bucks on electricity. I’m not a fan of waste, and most of our electricity in this country has external environmental and healthcare costs. There are asthmatics and others who are sicker because of our use of electricity and throwing away a few dollars seems less important than keeping someone from being sick. Not soapboxing here, just explaining some of my motivation. I don’t like wasted energy for a lot of reasons, and if we can all save a few bucks and make our air cleaner in the process (and maybe grow our weed faster!), that seems like a win-win.

Thanks again for the discussion.

1 Like

I believe if you were to say that a light had the ideal spectrum, that statement in itself would make this null. If it’s not providing the light you want, would be very difficult to call it the ideal spectrum right?

The veg time in hydro vs soiless vs soil is worth consideration. The flip side of that is the other characteristics that come along with each method. You’re certainly could find one that would work, but they’re not always ideal. In addition, I haven’t grown hydroponically, but it sure seems that they don’t mature any faster. So minimum time before flower isn’t going to change, just your plant size. Which goes back to being able to offset that with plant count as well. And that takes you into the legality of things. If you’re limited by plant count, them maybe you’re willing to for go how efficient your grow is in order to produce as much as possible from a limited amount of plants.

You and I look at this on a similar level, but that’s not within everyone else’s ideals. And then there’s the up front cost as well. My lights will gain around 10% in efficiency if I ran them at half the output current. But I’m not about to drop an extra $100 per square foot to do it. Just like a ton of arguments could be made which is the best way to do it. An aero grow sog style from clone under a bunch 50 watt quantum boards sounds like it would be pretty efficient to me. But not only are you talking about some really expensive components, there’s a lot of moving parts in that too. But you’re talking about probably the most efficient lighting available, low nute costs virtually no seed costs, and fastest time to harvest. You would probably have to tweak on some veg times to see how much each additional week changed in yield vs cost.

It just seems like the average grower is much more concerned with start up costs, and a grow like that is far from cheap.

OK, right. How about I rephrase as “a complete spectrum range” which would mean some intensity/amplitude at each wavelength but perhaps not enough at some or too much at others. Is that possible? It’s easy to imagine.

Absolutely possible. Otherwise we could say spectrum didn’t matter, when in fact we know it does to at least some extent.

I think to go down the road of breaking down light spectrums you either have to make a small general statement and leave it at that, or you really have to dig deep. I don’t believe any of us are capable of getting too deep, but that’s another story. I still see people say plants don’t use green light, when there are several studies that support otherwise. Or people rant on how much the uv in their led panels has helped their grow, when the spectral distribution graph clearly falls short of 400nm.

Light spectrums are really a mess when it comes to horticulture. You’ll see 400nm to 700nm all day long. But we’re still above 25% relative quantum efficacy to around 720nm. And I will legit get a headache if we have another uv or ir discussion lol. It all pretty much comes down to how you value one study vs another and what you want to believe. By a lot of studies 3000k-4000k 80cri cobs aren’t ideal spectrum. Yet people are out there killing it with cobs in that range.

I like the idea of trying to find out how efficient lights are but think there are way too many variables to compare to find the best.

There’s not that many variables at all. All you need is the sphere data. Getting that is another story.

With some search time and scrolling through probably 50-100 pages of random internet flaming you can find it for Cree, citizen, bridgelux, luminous cobs. And Samsung stuff like quantum boards. As well as things like gavita lights, and probably some lec stuff too. But most of the other stuff those guys don’t even bother testing.

Some of the more reputable light manufacturers will give it to you as well. Fluence bioengineering, pacific light concepts, horticulture lighting group, and I believe I even seen it for the latest spectrum king light. There may be others too that I’m not aware of.

The one thing I will say is that sphere testing isn’t cheap. So if you can’t find it, that means the particular light manufacturer didn’t want to invest that much into their product. Or they did, they just don’t want to tell you the results. I’ll let you draw your own conclusions from there.

For a good idea about the spectrum you want, you can find it on line easily. Not just the absorption peaks for the two chlorophylls, but one that shows chlorophyll synthesis and other metabolic reactions that need light. From that, you get an idea of the spectrum to look for in a light. Lots of blue and red, some green and far red, but not as much. What is not very good is early white LEDs that had nothing but blue and yellow.

COBs have an advantage over white LEDs because manufacturers go to some trouble to dial in a mix of phosphors to yield a particular color temp (and thus spectrum). Colored LEDs are good because they have a very narrow bandwidth and you can use them to fine tune your spectrum. You can also do tricks like mixing different color temp COBs to add some spectra together.

In laymen’s terms, what’s sphere testing?

1 Like

In laymen’s terms. It’s a closed sphere with ultra reflective surface that catches all of the light released by the light source. As in not a spot measurement, all of it from every wavelength. Not like a photometric testing device that’s weighted to specific wavelength.

When it comes to horticulture applications, photometric measurements don’t really mean much. They are weighted to how bright a light looks to us, and that’s what @1BigFella was talking about. In an integrated sphere they have the capability of testing for radiometric energy, which is what plants care about. So you can get a breakdown of total photons, photons in the par range, and amount of photons at each specific wavelength.

One thing that kind of stands out with testing of hid bulbs, is that a sphere catches all 360 degrees of light. And when they’re in a horticulture style fixture, more than half of that light has to be reflected. Reflection isn’t 100% efficient, so they usually test out a tad higher than is reasonably expected to see in a grow.

1 Like

Learn more and more everyday, thanks

1 Like

U bet! There’s some pretty good videos on YouTube if you’re interested.

1 Like