How Your Lighting System Can Affect Air and Leaf Temperature

HortiLED-Top-PL-Light-Systems-featureAs growers educate themselves more on different lighting technologies, one of the more common issues they must address is the effects that different light sources can have on temperature and crop production.

In a recent post on P.L. Light Systems’ educational blog page, Sales Manager Eric Moody notes that anyone familiar with HPS (high pressure sodium) and LED lighting should understand that you need to maintain a certain distance from your crop to the hot HPS luminaires, while cooler LEDs can be mounted closer to the canopy. Both of these lighting applications have their pros and cons depending on the crop you are growing.


“We also know that using LEDs can save an indoor grow in cooling costs, while HPS lamps can save a growing operation in heating costs (especially for those in Northern climates),” Moody says.

However, there is a common misconception when calculating efficiencies. When you calculate the load for heating/cooling equipment, you use a measurement of energy called British Thermal Units (BTUs). A BTU is equal to the amount of energy used to raise the temperature of one pound of water one-degree Fahrenheit.  When factoring in the heat given off by a luminaire, the BTUs are calculated off of the total wattage of the luminaire. If you have a 600 Watt HPS lamp, the BTUs for that luminaire are the same as a 600 Watt LED. A single watt of power is equal to 3.41 BTUs so a 600W luminaire will produce 2,046 BTUs. This is true for both the LED and the HPS luminaires.

Check out the complete post from Moody to get answers to the following questions:

If they both produce the same number of BTUs, then where is the savings in cooling an LED over an HPS?

If both technologies produce the same number of BTUs per watt then why does it feel so much warmer under the HPS than the LEDs? Why can I mount my 600W LED so much closer to the crop than the 600W HPS?

What are the pros and cons of radiant heat for my grow?

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It makes no sense on the same number of BTUs. Actually the LED fixture may reach higher efficiacy, say 2.2-2.5umol/J, than HPS. The majority portion in HPS spectrum is the far red or infrared and is harmful for plants.

Avatar for Jeff Mastin Jeff Mastin says:

@plantlighting, a good LED light will convert more of the electricity used into photons of light as you said, but ultimately almost all of this energy becomes heat again in the end. For example, say 50% of the power entering the LED light becomes photons. The other half immediately becomes heat. The other half becomes photons. Most of these get absorbed by the plants, while some are absorbed by greenhouse structures, and a very small amount reflect out of the greenhouse. Of the absorbed light, a very small amount, e.g. <5%, becomes contained in the plant's biomass. The rest is lost as heat again due to inefficiencies in photosynthesis and heat generation from respiration. So except for the stray photons that escape a greenhouse and the small fraction that becomes part of the biomass, all energy in becomes heat regardless of the light.

Now, LEDs can make the same amount of light with less energy in, which will reduce the heat load for the same light levels (but not different power inputs). The other big difference as you mentioned is the IR radiation from HPS. Surfaces under the HPS receive radiant heat and become warmer than the air temperature. That's not more BTUs of heat being made, but it makes the plants warmer than the air so the end result in the plant is a similar effect to a warmer room due to more heat being made when comparing to LEDs that dissipate their heat right into the air.

Hope that's more helpful than confusing…
-Jeff Mastin