Does LED out-yield HPS?
Not consistently. At equal canopy light levels, the trials do not show a universal LED yield advantage, and in most of them HPS grew as much flower or more.
| Trial | Comparison | Main finding |
|---|---|---|
| Magagnini 2018[3] | HPS vs two LED spectra; one THC-dominant clone; equal light | HPS plants were taller, with more dry flower in one of two runs |
| Westmoreland 2021[1] | HPS vs four LEDs; one CBD hemp cultivar; equal light | Dry flower yield fell 12% from the least blue lamp (HPS) to the most blue LED. The most efficient LED gave 4.6% less flower than HPS per square meter and 27% more per dollar of electricity |
| Danziger and Bernstein 2021[4] | HPS vs a white LED and two blue-plus-red LEDs; three cultivars | The highest flower yield came under an LED, the one with equal parts blue and red; the lowest came under the white LED |
| Morello 2022[5] | HPS vs five single-color or two-color LEDs; one THC/CBD cultivar; equal light | HPS grew the most flower: 134 g fresh weight per plant, against 76 g under blue light alone |
| Rodriguez-Morrison 2021[6] | LED only; one cultivar; a fifteen-fold range of light intensity | Dry flower yield rose in a straight line with intensity, from 116 to 519 g per square meter. No HPS arm |
The gaps mostly follow blue light. A sodium lamp emits very little of it, and blue keeps plants short with smaller leaves, so a bluer fixture tends to build a compact plant that catches less light. The Danziger result runs the other way, a reminder that cultivar matters. Why color shapes a plant at all is where our photobiology series begins.
Five small trials are not many, and two had lighting-industry money behind them. The Magagnini trial was funded by the maker of its LED fixtures, which employed one of its authors. The Utah work had support from a lighting company that, the paper says, played no part in the study.
The last row matters most to a small grower: yield tracked the photons delivered, with no plateau at the highest level tested. So a grower who swaps a tired sodium bulb for an LED and harvests more may simply have changed how much light reaches the canopy. In our view the photon count and its cost are the real LED vs HPS comparison.
Why do growers still use HPS?
Because the fixture is cheap, it throws a lot of light, and its low-blue spectrum happens to suit flowering.[1]
Heat is the other reason, and the “LEDs run cool” line gets it wrong. Nearly every watt a fixture draws ends up as heat in the room, whatever the lamp. An LED making the same light draws fewer watts, so there is less heat. LED fixtures also release more of their waste heat through convection and generally deliver less infrared radiation to the canopy than HPS.[7]
An energy-balance model from 2015 put leaves under LEDs about 1.3 °C cooler than leaves under HPS, and giving off less water, an effect its authors called smaller than commonly assumed.[7] Its leaf measurements came from vegetable crops, not cannabis. Either way, a room moved to LED needs its temperature and humidity setpoints checked again, and in a cold room the radiant heat of HPS is doing useful work.
How many LED watts replace a 1000 W HPS?
It depends on the two fixtures. As an example, going by the efficacies measured in the Utah trial, about 685 W of its most efficient LED would match the light of its 1,000 W sodium lamp.[1]
The number behind that is photon efficacy, the light that comes out per unit of electricity: 1.72 micromoles of photons per joule for the trial’s double-ended sodium lamp and 2.51 for that LED. A 2020 review of independently tested fixtures found LEDs both below and above that figure, and fixtures have changed since.[8] Do the same sum for the fixture you are considering, and check actual power draw and independently measured photon output rather than advertised replacement wattage. For how much light a crop needs and what kind, see our page on lighting quantity and quality.
What an LED vs HPS swap saves is the gap in watts times your hours and your electricity price, before any saving on cooling. Against it sits the purchase price: a 2019 review put LED fixtures at several times the cost of HPS, so check current prices.[2] That review called both systems viable and the ideal one “difficult to determine”, which is still fair.
Does the light change cannabinoids and terpenes?
It can, and the useful distinction is between concentration and total: spectrum can affect the cannabinoid percentage in the flower and the amount a plant produces differently. The evidence is cultivation trials on a handful of cultivars.
Where there was an effect, more blue gave a smaller plant with more concentrated flower. In the Magagnini trial, THC was 9.5% under HPS and 15.4% under the bluer LED in the first run, with no significant difference in total cannabinoid per plant.[3] In Morello’s, blue light alone gave the highest THC percentage, 10.2%, while HPS gave the most THC per plant, 2.54 g.[5] The Utah hemp trial found no effect of spectrum on CBD or THC concentration.[1]
None of this means plants use only red and blue. Green photons are at most about 10% less efficient at photosynthesis than red ones, and they reach deeper into a canopy.[8] Our page on how light spectrum affects cannabis covers the photoreceptors involved.
Terpenes have the thinnest evidence. In Morello’s trial, terpene concentration was 25.8 mg per gram of flower under HPS, higher under the two blue-rich LEDs (29.4 and 28.5) and lower under red and amber (20.3 and 18.1), while HPS produced the most per plant because its plants were bigger.[5]
In the intensity trial, terpene concentration rose from 8.85 to 12.7 mg per gram between the dimmest and brightest treatments.[6] That fits the reasoning behind subcanopy lighting, which puts photons where the top fixture doesn’t reach.
Daniel Hopper, cultivation manager at Silver State Relief in Nevada, told MJBizDaily in 2017 that full LED lighting raised terpene content in three of his four cultivars and made no difference in the fourth, in experiments he described as small and not yet repeated. That matches the trials: light moves terpenes, by amounts that depend on the plant.
So which should I choose?
It depends on what you already own and what your power costs. If you are buying new and electricity is expensive, a tested LED is the better buy, and we would not pay extra for an exotic spectrum over a higher efficacy figure. If you already run HPS in a cool room where its heat is welcome, the trials give you no yield reason to hurry. If the room overheats or the bill hurts, replace the lamp and recheck your climate setpoints the same week. Either way, measure the light at the canopy before and after, because that is the number the plants respond to. If you are choosing a first lamp rather than comparing trials, our plain-language guide to cannabis lighting covers how bright, how many hours and how close.
References
- Westmoreland FM, Kusuma P, Bugbee B. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS One. 2021;16(3):e0248988.
- Eichhorn Bilodeau S, Wu BS, Rufyikiri AS, MacPherson S, Lefsrud M. An update on plant photobiology and implications for cannabis production. Front Plant Sci. 2019;10:296.
- Magagnini G, Grassi G, Kotiranta S. The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L. Med Cannabis Cannabinoids. 2018;1(1):19–27.
- Danziger N, Bernstein N. Light matters: Effect of light spectra on cannabinoid profile and plant development of medical cannabis (Cannabis sativa L.). Ind Crops Prod. 2021;164:113351.
- Morello V, Desaulniers Brousseau V, Wu N, Wu BS, MacPherson S, Lefsrud M. Light quality impacts vertical growth rate, phytochemical yield and cannabinoid production efficiency in Cannabis sativa. Plants. 2022;11(21):2982.
- Rodriguez-Morrison V, Llewellyn D, Zheng Y. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Front Plant Sci. 2021;12:646020.
- Nelson JA, Bugbee B. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS One. 2015;10(10):e0138930.
- Kusuma P, Pattison PM, Bugbee B. From physics to fixtures to food: current and potential LED efficacy. Hortic Res. 2020;7:56.
Updated November 2026: this article was rewritten by the editors around the published LED-HPS trials. Originally published 22 November 2019 by Caleb Summeril as a short list of the pros and cons of each lamp.
Edited by Nani Frenkel, chief editor.
