# Calculate it properly: AC and cooling load for a grow tent or room

> Your cooling load is every watt you plug in, times 3.412 BTU/h, plus heat through walls and from people. Lights, drivers, fans, pumps and the dehumidifier all count. Plants don't add heat; they turn some of it into humidity, and a dehumidifier turns it back into heat. A sealed 10×10 ft room with about 3,300 W of LED needs roughly 16,000 BTU/h (1.3 tons) of cooling with dehumidification, before a 10–20% margin. Prefer a mini-split to a single-hose portable unit.

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## Questions and answers

**How many BTU do I need to cool my grow room?** Add up every watt running in the room (lights, drivers, [fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md), pumps, [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md), controllers) and multiply by 3.412 to get BTU/h, then add wall and ceiling gains and about 450 BTU/h per person while you're inside. A sealed 10×10 ft room with about 3,300 W of [LED](https://genetixbay.com/knowledge/glossary/led.md), 300 W of fans and a 1,000 W dehumidifier comes to about 16,000 BTU/h, or 1.3 tons. Add a 10–20% margin.

**Does a dehumidifier heat up a grow room?** Yes. A dehumidifier adds all its electrical watts as heat (3.412 BTU/h per watt), and condensing water releases about 2,320 BTU of latent heat per litre (1,050 BTU/lb) back into the room as sensible heat. That heat isn't new: the plants took it from the room when they transpired the water. But your AC now has to remove it as sensible heat, so size the AC with the dehumidifier running.

**How many tons of AC for a grow room?** One ton of cooling is 12,000 BTU/h (3.52 kW). Divide your total heat load in BTU/h by 12,000. A 4×4 ft tent with about 600 W of LED and fans plus a small dehumidifier adds about 2,600 BTU/h (0.2 ton); a sealed 10×10 ft room with about 3,300 W of LED is about 16,000 BTU/h (1.3 tons) before a 10–20% margin.

**Is a portable air conditioner good for a grow room?** A single-hose portable air conditioner is a poor choice for a grow room. It blows room air outside to cool its condenser, which pulls the same amount of warm, unconditioned air back in through gaps, so its real cooling is lower than the label suggests. US energy rules now rate portable units with this infiltration counted. A dual-hose portable is better; a ductless mini-split is better still for a sealed room.

**Should I oversize my grow room AC?** Only a little. A 10–20% margin covers hot days and growth, but a much larger unit cools the air quickly, shuts off, and removes less moisture, so humidity climbs. In a grow room that matters most in flower. A variable-speed (inverter) mini-split handles the swing between lights-on and lights-off better than an oversized single-speed unit.

**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](https://genetixbay.com/knowledge/tools/#ac). Planning the whole tent at once: [whole-tent planner](https://genetixbay.com/knowledge/tools/#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](https://genetixbay.com/knowledge/glossary/chiller.md), 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](https://genetixbay.com/knowledge/glossary/transpiration.md), 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.

**Adding up a cooling load.** List every device: Lights, fans, pumps, dehumidifier → × 3.412 (BTU/h) → + walls and people: ~450 BTU/h per person → ÷ 12,000 (tons) → + 10–20% margin. Source: [1][2]

## Worked example: a sealed 10 × 10 ft room

Assumptions (labelled; change them to your own): flower at about 900 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) with a top-tier 3.0 [µmol/J](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) LED (about **3,280 W**, see [lighting sizing](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)); **300 W** of [circulation fans](https://genetixbay.com/knowledge/glossary/oscillating-fan.md), 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](https://genetixbay.com/knowledge/equipment/calc-dehumidifier-sizing.md)); 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].

**Chart: Where the heat comes from in a sealed 10 × 10 room.** In the worked example the LED lights add about 11,190 BTU/h, the dehumidifier about 3,410, fans and controllers about 1,020, and walls plus one person about 750: about 16,370 BTU/h, or 1.36 tons.

| 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 |

Source: Worked example table above; watts × 3.412 [1]; people [2].

**Sealed 10 × 10 room, worked example.** ~16,370 BTU/h: Total heat load; 1.36 tons: Before margin; 1.57 tons: With 15% margin; ~6,500 BTU/h: Latent share the AC alone would struggle with. Source: Worked example above [1][2][3].

**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). So in flower the AC alone can't keep [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) 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](https://genetixbay.com/knowledge/glossary/hps.md) 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](https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.md)).

**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](https://genetixbay.com/knowledge/glossary/negative-pressure.md), 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](https://genetixbay.com/knowledge/equipment/calc-dehumidifier-sizing.md)).

## Sources

1. Thompson A, Taylor BN. 2008. NIST Special Publication 811, Appendix B (1 Btu_IT/h = 0.293 071 1 W; 1 ton of refrigeration = 12,000 Btu/h). National Institute of Standards and Technology. https://www.nist.gov/pml/special-publication-811
2. ASHRAE. 2021. ASHRAE Handbook: Fundamentals, Chapter 18, Nonresidential Cooling and Heating Load Calculations (heat gain from lighting, equipment and occupants; occupant heat gain table). https://www.ashrae.org/technical-resources/ashrae-handbook
3. NIST Chemistry WebBook. Water: enthalpy of vaporization (about 44 kJ/mol near 25 °C, about 2,440 kJ/kg). https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185
4. US Department of Energy. Room air conditioners (sizing: an oversized unit cools quickly but removes less humidity). Energy Saver. https://www.energy.gov/energysaver/room-air-conditioners
5. US Department of Energy. 2016 (1 June). Energy Conservation Program: Test Procedures for Portable Air Conditioners, final rule (Appendix CC, including infiltration air heat transfer for single-duct units and seasonally adjusted cooling capacity). Federal Register 81(105). https://www.govinfo.gov/content/pkg/FR-2016-06-01/html/2016-12446.htm
6. US EPA. Section 608 Technician Certification (refrigerant handling for split systems). https://www.epa.gov/section608