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:
- PPF: Photosynthetic Photon Flux. Measure of how much PAR is produced per second by the light.
- 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.
- 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.