Crop scientists at Utah State University recently released a study that is generating a lot of buzz in the indoor gardening, hydroponics, and greenhouse market. The scientists measured the photosynthetic (400-700 nm [PAR range]) photon efficiency and photon distribution patterns of double-ended HPS lights, LEDs, Fluorescents, Induction, Ceramic Metal Halide, and standard mogul base HPS lights [they measured popular indoor garden lights such as the Gavita Pro 1000w Complete Double Ended, ePapillion 1000w Double Ended, LSG Violet, SPYDR 600, LSG red/white, Illumitex NeoSol 300w NS, Lumigrow Pro 325, California Light Works Solar Storm 400, Black Dog BD360 Universal Series, Apache Tech AT120WR, iGrow 400w Induction Light, Lumigrow ES 330, and HydroGrow Sol 9, Cycloptics Ceramic Metal Halide, Fluorescent T8 fixtures, and mogul base 1000w HPS fixtures]. The study also took initial cost of the fixture and electrical costs to conclude that LED and double-ended HPS lights were the most efficient lights out of the bunch, with double-ended HPS slightly ending out LED.
The way this test calculates the economics of a lights efficiency is by factoring in the initial capital cost of a fixture and the cost of electricity to operate it over a 5 year period and comparing that to the amount of photosynthetic photons (light that plants are known to use to photosynthesize) they deliver.
Over a 5 year timeline, the study found that when you factor in electricity and fixture cost per mole of photons, LED lights are approximately 2.3 times more expensive than HPS fixtures to operate. This brings about another interesting question, which is how will that multiplier of 2.3 change as LED technology keeps progressing? As LED technology keeps getting less expensive and more efficient, by what year will that 2.3 multiplier dip below 1 making LEDs more efficient to run than HPS lights, in turn totally changing the standard for indoor grow lights from HPS to LED.
The hard part about doing a test like this is that measuring light efficiency can never be absolute. What I mean by that is that there is no one light that does everything a plant or specific garden setup needs. Some plants grow larger and taller than any one fixture can satisfy, and some are small enough that much light may go unused. Also, garden setups may have long skinny rows, or may be designed in some way that's completely different. For example, LED lights have optics that can be manipulated to have a specific coverage area, while HPS lights need reflectors to perform that function, and reflectors are often less precise. So although this test found that LEDs are more expensive over a 5 year period when all emitted photon radiation is assumed captured by plants, when a more narrow region is considered to be captured below the fixture (for example on flood tables or benches) some LEDs in the test actually had a lower cost per photon than HPS fixtures. So a HPS light emits more light, but the LED lights have the potential to be better at taking their energy input and directing it over a small, specified region. Results like this don't make choosing a light quite as easy as we had hoped.
We have to remember when looking at studies like this that they don't tell the whole picture for most of us indoor growers, and we can't expect it to. Tests like this need to set parameters, assumptions, and measure variables accurately. Throwing more variables into the equation, such as cooling the environment and its associated costs, throws in added complexity. If you're feeling extra nerdy, read through the PDF of the academic write up below to view this information in all its detail. We look forward to more universities providing insightful information like this to help us make important purchasing decisions for our gardens rather than having to rely on marketing claims, which can often be misleading and inaccurate.
Lastly, one of the coolest and most useful things to come out of this test is a cost calculator that helps you compare light fixture cost over a 5 year period. A great resource that you can download here: Five-Year Cost Calculator for Grow Light Fixtures Excel File
This is Nate from Growers House. Happy Growing.
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