Dehumidification Load in Indoor Cannabis Cultivation

Where the water comes from

An indoor grow is a humidifier with a crop in it. Water goes onto the root zone, the plant pulls it up, and the leaves release most of it as vapor. Engineering estimates of that fraction vary with medium, irrigation practice, and how much runoff you capture: an HPAC Engineering analysis puts transpiration at about 70 percent of total irrigation water returned to the air [hpac-latent-2023], while Denver's cannabis energy guide says simply that the water applied to plants is transpired by the plants and then has to be removed from the space [denver-bmp-energy-2019]. Manufacturer sizing guidance converts that into a canopy rule of thumb of 0.5 to 2 pints of moisture per day for every square foot of canopy, with the grower adjusting for their own conditions [quest-dehum-101].

The transpiration math, step by step:

1. Total daily irrigation in gallons.
2. Multiply by 8 to get pints (1 gallon is 8 pints).
3. Multiply by the transpired fraction. Use 0.7 for a conservative estimate of load, 0.9 or higher if you run hydroponics with little runoff, and check it against your own drain volumes.
4. Note when it happens. Plants transpire mostly during lights-on, so a 12-hour photoperiod concentrates most of the daily water into 12 hours.

Southern California Edison's market study reports indoor flowering irrigation in the range of 0.096 to 0.16 gallons per square foot per day [sce-etp-2021]. At 0.13 gallons and an 80 percent transpired fraction, that is 0.13 × 8 × 0.8 = about 0.83 pints per square foot per day, comfortably inside the manufacturer rule of thumb.

Latent versus sensible: why this is a separate load

HVAC engineers split the work of conditioning a room into two parts. Sensible load is heat you can measure with a thermometer: fixture watts, motors, people, heat through the walls. Latent load is the energy tied up in water vapor. When a plant evaporates a pint of water it absorbs heat from the room to do it, converting sensible heat to latent heat and cooling the air while loading it with moisture [hpac-latent-2023]. That heat comes back out when a coil condenses the water.

Conventional air conditioners are built for buildings where most of the load is sensible. The HPAC analysis describes this with the sensible heat ratio, the share of a unit's capacity that goes to sensible cooling; most packaged equipment performs well at ratios of 80 percent and up, while grow rooms often operate well below that [hpac-latent-2023]. A plant-filled room can have more moisture to remove than heat, and a standard AC unit will overcool the space trying to wring the air out, then need reheat.

The lights-off period is where this bites. Fixtures are off, so sensible load collapses, but the plants are still respiring and the room is still wet. Denver's guide notes that the warmer the room can be kept during lights-off, the more efficiently dehumidification equipment operates [denver-bmp-energy-2019], which is why facilities that moved to LED and lost the free HPS heat sometimes add reheat. The HVAC sizing page covers equipment selection for low sensible heat ratios.

Equipment choices and what they cost to run

Unit conversion for comparing the ENERGY STAR and manufacturer figures: 1 liter is about 2.1 pints, so 3.30 liters per kWh is roughly 7 pints per kWh and 2.01 liters per kWh is roughly 4.2. Note the rating conditions differ, so treat the comparison as approximate.

Two practical points. First, efficiency is quoted at a rating point and drops as the room gets cooler and drier, so a unit that reads 8 pints per kWh on the spec sheet does less in a 68 degree lights-off room. Second, oversizing is not free: a unit that short-cycles wastes the start-up energy of every cycle. Manufacturer guidance is to match capacity to actual canopy water loss and distribute several units rather than one large one [quest-dehum-101].

A worked example: a 5,000 square foot flower room

Every number here is an input we chose. Substitute your own irrigation logs and equipment specs.

Assumptions:

Water load: 5,000 × 0.15 = 750 gallons per day = 6,000 pints. Times 0.8 = 4,800 pints per day to remove.

Equipment count: 4,800 ÷ 506 = 9.5 units at nameplate. With a 25 percent margin for post-irrigation spikes and derating in lights-off, plan on 12 units.

Energy, efficient case: 4,800 ÷ 8 = 600 kWh per day, about 18,000 kWh a month. At 14.19 cents: about 2,550 dollars a month.

Energy, portable case: 4,800 ÷ 4.2 = 1,140 kWh per day, about 34,300 kWh a month, about 4,870 dollars a month.

Demand: if all 12 overhead units run at once, 12 × 2.7 kW = 32 kW. At 12 dollars per kW that is about 390 dollars a month, and more importantly it lands on top of the lighting peak if the units all start after lights-on. See demand charges explained.

The gap between the two energy cases, roughly 2,300 dollars a month for one 5,000 square foot room, is the cost of equipment choice alone, before any change in the water load. Reducing irrigation runoff, tightening the envelope so outside humidity stays outside (see weatherization and building envelope), and raising lights-off temperature all reduce the load itself. The facility energy cost calculator lets you run the whole facility.

How it shows up on the bill

Dehumidification is invisible on a utility bill because there is no line for it. It shows up in three places:

• Around-the-clock kWh. Lights run 12 hours; dehumidifiers run 24. In the Northwest Council's producer survey, cooling was 15 percent of total facility electricity, with ventilation, dehumidification, and pumps making up most of the rest of the 33 percent non-lighting share [nwpcc-cannabis]. Facilities in humid climates report higher shares; Denver's guide says climate control can be 50 percent or more of an indoor facility's consumption [denver-bmp-energy-2019], and the Colorado State life-cycle study found HVAC held the largest energy demand once local climate was included [summers-2021].
• Demand. Compressors starting together after irrigation or lights-on set the interval peak. Staggering unit start-ups by a few minutes and sequencing irrigation across rooms spreads the load across intervals.
• Load factor. Because it runs in both photoperiods, dehumidification raises the ratio of average to peak load, which suppliers price favorably. A facility with a flatter profile gets a better fixed-price quote than one with a spiky one, all else equal.

For the whole-facility view, the kWh per square foot benchmarks page shows how lighting and HVAC split in published surveys, and the LED vs. HPS page explains why cutting lighting watts changes the sensible load a lot and the latent load hardly at all.