Estimating Grow Light Heat Load for HVAC Sizing: A Room-Level Calculation
Every watt of lighting power becomes heat inside a closed room, so the cooling load follows from fixture power and efficacy rather than from the fixture datasheet’s marketing line
A grow room's cooling load is decided, in large part, by the lights. That is not a figure of speech. In a sealed room, essentially all electrical power drawn by the lighting system ends up as heat inside the envelope, and the lighting schedule usually coincides with the hours when cooling is already hardest. Yet lighting heat is routinely estimated from a rule of thumb rather than from the two numbers that actually determine it.
This article sets out a room-level calculation, converts between the units an HVAC engineer will ask for and the units a lighting buyer already has, and works through a 500 square metre room so the arithmetic is visible.
The conversion that anchors everything
Heat and electricity are the same quantity in different units. The U.S. Energy Information Administration gives the standard factor: one kilowatthour equals 3,412 Btu. Divide by the hour and one kilowatt of continuous electrical input equals 3,412 Btu per hour of heat.
That single line converts any lighting load into a cooling load. A 600 W fixture running at full power adds 600 divided by 1,000, multiplied by 3,412 - about 2,047 Btu per hour - to the room. A 720 W fixture adds about 2,457 Btu per hour. Nothing about the light's spectrum or its efficiency changes this, because the calculation starts from input power, and input power is what the meter measures.
Why efficacy matters even though the conversion does not use it
The conversion above answers a different question from the one most buyers ask. It answers "what heat does this fixture add?" Efficacy answers "how many photons do I get for that heat?"
Photosynthetic photon efficacy is expressed in micromoles per joule, and it is measured at the input. A fixture delivering 3.0 micromoles per joule produces the same photon flux as a 2.0 micromole per joule fixture while drawing two thirds of the power - and therefore adding two thirds of the heat. The ratio is exact, because the denominator is the same electrical watt in both cases.
That relationship gives a second way to size, starting from the photon flux the crop needs rather than from the fixture count. If a room must receive a given photosynthetic photon flux density over a known area, the required photon flux is fixed, and the power needed to deliver it is that flux divided by efficacy:
The table is read as follows: at an efficacy of 2.0 micromoles per joule, every 1,000 micromoles per second of delivered photon flux costs 500 W of electrical input and adds about 1,706 Btu per hour of heat; at 3.0 micromoles per joule the same photon flux costs 333 W and adds about 1,137 Btu per hour. The efficacy values are illustrative points across the range commonly quoted for horticultural LED fixtures, not a claim about any specific product or about a programme threshold.
The practical consequence is that upgrading efficacy reduces the cooling load per unit of delivered light by the same percentage. This is the part of an LED retrofit business case that is often left out: the saving is not only on the lighting line of the electricity bill but also on the cooling line, because the heat rejected per photon falls.
A worked room
Take a 500 square metre room, 5,382 square feet, with 24 fixtures at 720 W each.
- Connected lighting power: 24 x 720 = 17,280 W, or 17.28 kW
- Lighting heat: 17.28 x 3,412 = 58,960 Btu per hour
- In tons of refrigeration at 12,000 Btu per hour per ton: 4.91 tons
Two checks are worth running on that figure. First, power density: 17,280 W over 5,382 square feet is about 3.2 W per square foot of lighting, which tells an HVAC engineer immediately whether the room is lightly or heavily lit. Second, the share of total load: if the room's envelope and equipment loads add another 3 tons, lighting is roughly 62 percent of the sensible cooling load - a normal ratio for a well-insulated production room, and the reason lighting dominates summer sizing.
Sensible, latent and the dehumidification trap
In a sealed room, lighting contributes almost entirely sensible heat. It does not add moisture, so it does not raise the latent load directly. But it does raise leaf temperature and transpiration, and transpiration is the dominant latent load in a plant canopy. A room sized only against lighting heat and equipment heat will usually be short on dehumidification capacity, not on sensible capacity.
Get Horticulture Growlights’s stories in your inbox
Join Medium for free to get updates from this writer.
The practical implication for sizing is to keep the two loads separate in the calculation and to specify the dehumidification requirement from the irrigation and transpiration model, not from the lighting figure. Lighting heat sets the sensible tonnage; the crop sets the moisture removal.
Where the estimate goes wrong
Four errors account for most of the gap between calculated and installed capacity.
Using the datasheet's "low heat" claim. Marketing language about LEDs running cooler refers to the fixture surface or to the absence of a hot arc tube. It does not mean the room receives less heat. For equal input power, the room receives the same heat regardless of technology. For equal delivered photons, LED fixtures usually receive less, which is the real and defensible claim.
Using nameplate power when the fixtures are dimmed. A room held at 70 percent output draws 70 percent of the power and adds 70 percent of the heat. Sizing for nameplate is conservative and safe for peak design, but it overstates annual cooling energy, sometimes badly.
Forgetting driver and control losses. Efficacy figures are normally reported at the input, so driver losses are already inside the wattage. If a project compares a bare board figure against a complete fixture figure, the comparison is not like for like, and the heat estimate inherits the error.
Ignoring the schedule. Cooling design conditions are usually set for late afternoon in summer. A lighting schedule that runs through that window stacks lighting heat on top of peak outdoor conditions. The same room lit only at night presents a materially lower design load, which is a legitimate reason to revisit both the tariff analysis and the HVAC sizing together.
FAQ
Does a 1,000 W LED fixture really produce 3,412 Btu per hour?
At full power, yes. One kilowatt of electrical input equals 3,412 Btu per hour, and in a closed room that energy leaves as heat. The figure is a unit conversion, not a product characteristic.
Can I use the fixture's PPE to reduce the cooling load I specify?
You can, and you should, provided you compare fixtures at equal delivered photon flux rather than at equal wattage. Higher efficacy means fewer watts for the same photons, and therefore less heat. If the design keeps the same installed wattage and simply uses more efficient fixtures, the room gets more light and the same heat.
How much margin should be added to the calculated lighting load?
That is an engineering decision for the mechanical designer, and it depends on the room's other loads and the acceptable temperature excursion. What matters for the lighting input is that it be given as an explicit number - connected power, expected dimming level, and operating hours - rather than as a rule of thumb per fixture.
Conclusion
The calculation is two steps. Convert input power to heat using 3,412 Btu per kilowatthour, then decide whether the design question is about installed power or about delivered photons. The first step sizes the equipment. The second decides how much of the electricity bill, and how much of the cooling plant, the lighting system is responsible for.
Getting the number onto one line, with the dimming level and schedule stated as assumptions, is worth more than any refinement of the rule of thumb it replaces. It also makes the efficacy case visible in the place where facility budgets are actually decided, which is the mechanical room rather than the fixture schedule.
Koray (Zhongshan Koray Opto-Electronic) has manufactured horticultural LED lighting for 17 years and holds more than 300 patents in China and abroad. Fixture specifications and test documentation are available at koraylights.com.
Sources
- U.S. Energy Information Administration, Units and calculators explained, sample Btu conversion factors, one kilowatthour equals 3,412 Btu: https://www.eia.gov/energyexplained/units-and-calculators/
- U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A, average price of electricity by end-use sector, data for July 2026: https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a
- DesignLights Consortium, Horticultural Lighting programme page, describing the Qualified Products List and the Horticultural Technical Requirements V4.0 effective April 18, 2025: https://designlights.org/our-work/horticultural-lighting/
- Greenhouse Grower, reporting on the DLC horticultural requirements and the U.S. Department of Energy recommendation that growers transition to LED lighting, projected to cut electricity use by at least 40 percent: https://www.greenhousegrower.com/production/new-horticulture-lighting-requirement-guidelines-under-consideration/
Assumptions used in the worked example, stated separately: 24 fixtures at 720 W each in a 500 square metre room, and an additional non-lighting sensible load of 3 tons. The efficacy values in the table are illustrative points, not product data.
