September 4, 2026
Under canopy lighting has moved from experiment to standard practice in a lot of commercial rooms, and the reason is simple. Overhead fixtures do a good job lighting the top 12 to 24 inches of a canopy and a poor job of anything below that. Adding light underneath puts photons on flower sites that were previously getting low to very low PPFD.
The problem is that under canopy fixtures have started to look interchangeable. Two systems can cover the same 4-by-4 area, mount the same way, and carry similar spec sheets while operating at wildly different power levels. A cultivator comparing 100 watts against 170 watts over the same footprint has a fair question: does the difference actually show up at harvest?
It does. And once you attach a dollar figure to it, the gap is larger than most people assume.
Start with the 1% rule
There is a rule of thumb that has held up across horticulture for decades: a 1% increase in usable light produces roughly a 1% increase in yield. It came out of greenhouse production, and cannabis follows it closely.
Rodriguez-Morrison, Llewellyn and Zheng found dry flower yield increasing linearly with canopy-level PPFD all the way to 1,800 µmol·m⁻²·s⁻¹, which works out to a DLI near 78 mol·m⁻²·d⁻¹. That is well above what almost any commercial room is running. Llewellyn’s 2022 work showed the same linear response through 1,000 µmol·m⁻²·s⁻¹. Sae-Tang’s 2024 study measured photon conversion efficiency holding steady at 0.25 grams per mole across intensity treatments, which is another way of saying the plant kept converting incremental photons into flower at a constant rate.
The practical read on all of this: cannabis has not shown a light saturation point inside the range commercial cultivators operate in. There is no shoulder on the curve where additional photons stop paying. Every photon you can get onto productive tissue is worth money, and you can calculate roughly how much.
That reframes the whole under canopy conversation. The question is not whether a fixture adds light. It is how many additional photons the system delivers to tissue that can use them, and what those photons return.
Under canopy is where the cheap photons are
The economics of adding light overhead get ugly fast. You are already at the intensity ceiling of your fixtures, you are competing with your own upper canopy for interception, and pushing higher means new gear, more circuits, and often more cooling capacity.
Under canopy is different. The lower third of a dense plant may be receiving 10 to 20% of what the top is getting. Those flower sites are not light saturated. They are light starved. Historically growers lollipopped that growth off because it could not earn its keep, which was the correct decision when the only light source was 6 feet overhead.
Put a fixture in there and those sites become productive. You are adding photons in the one part of the room where nothing is competing for them, and the plant material is already built and already fed. No additional nutrients, no additional water, no additional square footage. Just light.
That is why intensity underneath deserves more scrutiny than it usually gets. If you are going to the trouble of running power and hanging fixtures below the canopy, undersizing the light is the expensive mistake.
Run the numbers on 100 versus 170
Take a 4-by-4 position, a 12-hour photoperiod, a 60-day flower cycle, and a fixture efficacy around 2.7 µmol/J. That is a conservative efficacy for current under canopy hardware.
The 70-watt difference converts to about 189 µmol/s of additional photon output. Over a 12-hour day that is 8.2 moles. Across a 60-day cycle, roughly 490 additional moles delivered into that 4-by-4.
Now compare it to the overhead. A room running 40 watts per square foot puts about 640 watts over that same footprint, or roughly 1,730 µmol/s at the same efficacy. The extra 70 watts underneath represents about an 11% increase in total photons delivered to that position.
Apply the 1% rule and you are looking at an 11% yield difference between the two under canopy options. On a 45 gram per square foot baseline, that 4-by-4 produces about 720 grams. Eleven percent is roughly 78 additional grams per position, per cycle. At $3 a gram, that is $234.
The additional electricity costs $6. Seventy watts, 12 hours a day, 60 days, at twelve cents a kilowatt hour.
Even if you discount that hard, and you should, the ratio holds. Assume only a quarter of the incremental photons land on tissue that converts them. You are still at roughly $58 against $6 in power. Scale it to a 10,000 square foot canopy and the difference between the two fixtures is measured in hundreds of thousands of dollars a year against an incremental power bill under $20,000.
I want to be clear that these are modeled figures, not trial results. Interception rates vary by cultivar, plant density, defoliation strategy and mounting geometry. But the framework is what matters. Once you accept that photons convert to grams at a knowable rate, an under canopy fixture stops being a line item and becomes a return calculation. Nobody would install a 100-watt overhead fixture where 170 fit. The same logic applies underneath.
Watts alone will not get you there
None of this works if the fixture cannot convert power into photons efficiently or cannot get those photons onto plant material.
Efficacy is the first filter. A 170-watt fixture at 2.2 µmol/J produces fewer photons than a 100-watt fixture would at an efficacy nobody currently ships, but the point stands: check the µmol/J, not just the wattage. Two fixtures at the same power draw can differ by 20% in output.
Distribution is the second filter and it is harder to evaluate from a spec sheet. Overhead fixtures project down across an open area. An under canopy fixture sits inches from plant tissue and has to throw light sideways and upward into an irregular, dense structure. Concentrating photons into a narrow band accomplishes very little. You end up with a bright stripe of overexposed leaf and shaded flower six inches away. The output has to spread across the productive material.
Thermal design is the third, and it gets more demanding as power goes up. These fixtures live in restricted airflow, high humidity, and the direct path of irrigation, sprays and cleaning. A high-output fixture has to shed that heat without cooking the tissue it is sitting next to. Driver placement, housing construction and ingress protection all matter more here than they do overhead.
So the case for more watts is conditional. Additional power is only worth buying when it comes with the efficacy to convert it and the optics to place it. Adding wattage without those two things just raises your operating cost.
Where this lands
The JumpLights Catalyst® was designed against this exact set of constraints. High intensity underneath the canopy, distribution built for dense commercial plant structure, and a thermal and ingress design that holds up in a flower room rather than a test bench. It comes in multiple power levels and mounting configurations so the system can be matched to plant architecture and room layout instead of the other way around.
