Short answer, in plain words. Everything you plug in turns into heat, so your cooling job is simple to add up: every watt × 3.412 = BTU per hour. Add the lights, the drivers, the fans, the pumps and the dehumidifier. Add a little for hot walls and for you when you're in there. Divide by 12,000 to get tons. A sealed 10 × 10 ft room with about 3,300 W of top-tier LED comes to roughly 16,000 BTU/h (1.3 tons), then add 10–20% margin. Plants don't add heat; they hide some of it as humidity, and the dehumidifier hands it back. Established (physics); example values are labelled assumptions.
Use the calculator: AC and cooling calculator. Planning the whole tent at once: whole-tent planner.
What is the formula?
Total cooling load (BTU/h) = sensible gains + latent gains, and in a grow room nearly all of it is electrical:
- Every device: watts × 3.412 = BTU/h [1][2]. Lights (measured at the wall, so drivers and ballasts are included), fans, pumps, controllers, chillers, and the dehumidifier's own watts.
- People: about 450 BTU/h each while inside (about 250 sensible and 200 latent, for moderately active work) [2].
- Walls and ceiling: BTU/h ≈ area (ft²) × temperature difference (°F) ÷ R-value. In metric: W ≈ area (m²) × temperature difference (K) ÷ RSI.
- Tons = BTU/h ÷ 12,000. kW = BTU/h ÷ 3,412 [1].
What about the plants? They don't add energy. When leaves transpire, they turn some of the room's heat (mostly from the lights) into water vapour: about 2,450 kJ per kg of water, or roughly 2,320 BTU per litre (1,050 BTU/lb) [3]. That's a latent load: humidity instead of temperature. The total stays the same; only the split between "hot" and "humid" changes.
Why a dehumidifier adds heat. A dehumidifier adds all its electrical watts as heat, and when it condenses the water it releases that latent heat back into the room as sensible heat, about 2,320 BTU per litre [3]. So with the dehumidifier doing the drying, your AC must remove: all the electrical watts (dehumidifier included) as plain heat.
- List every deviceLights, fans, pumps, dehumidifier
- × 3.412BTU/h
- walls and people
- ÷ 12tons
- 10–20% margin
Worked example: a sealed 10 × 10 ft room
Assumptions (labelled; change them to your own): flower at about 900 µmol/m²/s with a top-tier 3.0 µmol/J LED (about 3,280 W, see lighting sizing); 300 W of circulation fans, controllers and a pump; plants using about 37 L (9.8 US gal) of water a day, about 90% of it in the 12-hour lights-on period (see dehumidifier sizing); a dehumidifier removing that at about 2.8 L/kWh, so about 1,000 W while the lights are on; an interior room with modest wall gains.
| Item | Watts | BTU/h |
|---|---|---|
| LED lights (wall watts) | 3,280 | 11,190 |
| Fans, controllers, pump | 300 | 1,020 |
| Dehumidifier (electrical) | 1,000 | 3,410 |
| Walls and ceiling (interior room; assumption) | ~300 | |
| One person, while inside | ~450 | |
| Total | ~16,370 (1.36 tons) | |
| With 15% margin | ~18,800 (1.57 tons) |
The transpired water (about 2.8 L/h in lights-on, about 6,500 BTU/h of latent heat) isn't a separate line: it came out of the light's heat and goes back in when the dehumidifier condenses it [3].
- LED lights
- Dehumidifier
- Fans, controllers, pump
- Walls and one person
Source: Worked example table above; watts × 3.412 [1]; people [2].
Show the numbers
| Room | LED lights (BTU/h) | Dehumidifier (BTU/h) | Fans, controllers, pump (BTU/h) | Walls and one person (BTU/h) | Total (BTU/h) |
|---|---|---|---|---|---|
| Sealed 10 × 10 room | 11,190 | 3,410 | 1,020 | 750 | 16,370 |
- ~16,370 BTU/hTotal heat load
- 1.36 tonsBefore margin
- 1.57 tonsWith 15% margin
- ~6,500 BTU/hLatent share the AC alone would struggle with
What if the AC does the drying instead? Without a dehumidifier the total is smaller (about 12,960 BTU/h including walls and one person), but about half of it is latent. Most comfort air conditioners are built to remove only about a fifth to a third of their capacity as moisture (check the unit's published sensible and latent capacity)being reviewed. So in flower the AC alone can't keep RH down. That's why sealed rooms pair an AC with a dehumidifier, or use equipment built for high latent loads.
Choosing the unit: at about 1.6 tons with margin, an 18,000 BTU/h (1.5-ton) variable-speed unit is close; a 24,000 BTU/h (2-ton) unit gives headroom but risks short-cycling at lights-off unless it is inverter-driven. Grower practice.
How much cooling for each tent size?
Same assumptions per square foot: top-tier LED at about 900 µmol/m²/s, fans scaled to size, plants using about 4 L/m²/day, a dehumidifier at 2.8 L/kWh. These are the heat each space adds to the room it sits in.
| Space | Lights (W) | Fans etc. (W) | Dehumidifier (W, lights-on) | Total heat (BTU/h) | Tons |
|---|---|---|---|---|---|
| 2 × 4 ft tent | ~260 | 40 | ~80 | ~1,310 | 0.11 |
| 4 × 4 ft tent | ~525 | 80 | ~160 | ~2,610 | 0.22 |
| 5 × 5 ft tent | ~820 | 100 | ~250 | ~4,000 | 0.33 |
| 4 × 8 ft tent | ~1,050 | 120 | ~320 | ~5,090 | 0.42 |
| 10 × 10 ft room | ~3,280 | 300 | ~1,000 | ~15,650 | 1.30 |
Add people, wall gains and a 10–20% margin. With double-ended HPS at 1.7 µmol/J for the same light, the lights alone need about 1.75 times the watts, so the load climbs accordingly (see LED vs HPS: the heat and power-bill myth).
Tents inside a house. A tent exhausting into the room it sits in hands all its heat to that room, so the house AC must remove it. A tent exhausting outdoors removes its heat, but the house must replace the air it throws out, and that make-up air brings outdoor heat and humidity in. Established (energy balance).
How much do walls and ceilings add?
Usually little for an interior room, a lot for an attic, garage or exterior wall. Gain ≈ area × ΔT ÷ R [2].
- Interior wall, 80 ft², R-4 (uninsulated drywall), 5 °F warmer outside the room: 80 × 5 ÷ 4 = 100 BTU/h.
- Ceiling under a hot attic, 100 ft², R-30, attic 120 °F vs room 78 °F (ΔT 42 °F): 100 × 42 ÷ 30 = 140 BTU/h. Uninsulated (about R-3): 1,400 BTU/h.
- Exterior wall in sun, 80 ft², R-13, ΔT 20 °F: 125 BTU/h; uninsulated: about 530 BTU/h.
Metric: W = m² × K ÷ RSI (RSI = R ÷ 5.678).
Mini-split or portable AC?
For a sealed room, a ductless mini-split. Avoid single-hose portable units. Supported.
- Single-hose portable: it blows room air across its hot condenser and out the window. That creates negative pressure, and the same volume of warm, unconditioned air leaks back in through every gap. US Department of Energy test rules now count this infiltration, which is why newer labels show a lower "seasonally adjusted" capacity than older ones [5]. In a sealed CO₂ room it also throws out your CO₂.
- Dual-hose portable: pulls condenser air from outside and returns it outside, so it avoids most of the infiltration problem. Better, still noisy and less efficient than a split.
- Mini-split: the hot condenser is outdoors; the indoor head only moves room air, so it doesn't depressurise the room or dump CO₂. Variable-speed models handle lights-on/lights-off swings well. Refrigerant work needs a certified technician (EPA Section 608 in the US) [6]; check local permit rules.
- Window unit: fine for some rooms; it vents like a dual-hose but needs a window and leaks some air around its seals.
Don't oversize much. A unit far bigger than the load cools fast, shuts off, and removes less moisture, so humidity rises [4]. Size for the load plus 10–20%.
Common mistakes
- Counting only the lights. Fans, pumps, dehumidifiers, chillers and controllers all add 3.412 BTU/h per watt [1][2].
- Forgetting the dehumidifier's heat, or sizing the AC with the dehumidifier off.
- Adding plant transpiration as extra heat. It's the light's heat, changed into humidity; count it when choosing how to dry the air, not as extra cooling load.
- Using lamp-rated watts instead of measured wall watts.
- Buying a single-hose portable for a sealed room and wondering why it's hot and the CO₂ is gone [5].
- Massively oversizing and fighting high RH in flower [4].
- Ignoring lights-off. The load drops by the lights' share; a single-speed unit may short-cycle and humidity can spike (see dehumidifier sizing).
