How much AC does my grow room need?
Every watt you plug in becomes heat: 1 W = 3.412 BTU/h, whether it's an LED, an HPS, a fan or a computer. Add up the wall watts of everything in the room, add people and the heat leaking through the walls, then add a margin. That's your AC size.
Your result
How it works, in plain words
Watts to heat
BTU/h = wall watts × 3.412. Four 1,000 W double-ended HPS draw about 1,045 W each, 4,180 W in all: 14,262 BTU/h before anything else.
People and walls
About 450 BTU/h per person working inside. Walls and ceiling: U-value × area × the temperature difference. A tent inside the house adds nothing here.
Tons
One ton = 12,000 BTU/h. Add about 20% margin and round up to a real size.
Sealed vs vented
A vented room swaps its air with the exhaust fan, which only works while the intake air is cooler than the room. A sealed room keeps the exhaust off, holds CO₂, and lets the AC and dehumidifier do all the work. Sealed is more control; vented is cheaper and simpler.
The dehumidifier
Count its watts. Don't add the heat it releases condensing water a second time: that energy came from the lights, when the plants evaporated it.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
BTU_PER_W | 3.412 BTU/h per W | Every watt drawn from the wall becomes 3.412 BTU/h of heat in the room, whatever the device | Established |
BTU_PER_TON | 12000 BTU/h per ton | One ton of cooling | Established |
PERSON_BTU | 450 BTU/h per person | Heat from a person doing light work (ASHRAE: about 250 sensible + 200 latent) | Established |
U_GOOD | 0.06 BTU/h·ft²·°F | Well-insulated room (about R-16 overall) | Established (typical assembly) |
U_AVG | 0.1 BTU/h·ft²·°F | Average insulated stud walls (about R-10 overall) | Established (typical assembly) |
U_POOR | 0.25 BTU/h·ft²·°F | Uninsulated framed walls (about R-4) | Established (typical assembly) |
U_SHED | 0.6 BTU/h·ft²·°F | Bare garage, shed or metal building | Estimate |
AC_MARGIN | 0.2 + | Cooling margin over the calculated load (homes keep within ~15%; grow rooms run near flat out, so 20% is common) | Grower practice |
Sources
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
- ASHRAE. 2021. ASHRAE Handbook: Fundamentals. Ch. 1 (psychrometrics: sensible heat of air, q = 1.08 × cfm × ΔT at standard air), Ch. 18 (heat gain from people, lighting and equipment), Ch. 21 (duct design: fitting and friction losses), Ch. 26–27 (thermal resistance of building assemblies). ASHRAE, Atlanta. https://www.ashrae.org/technical-resources/ashrae-handbook
- GenetixBay Knowledge Base. LED vs HPS: the heat and power-bill hype, and an LED buyer-beware guide (fixture µmol/J tiers, 85% canopy capture, watt-for-watt house advice, 1 W = 3.412 BTU/h). https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.html
- ACCA. Manual S: Residential Equipment Selection (cooling equipment kept close to the calculated load; oversizing short-cycles and dehumidifies poorly). https://www.acca.org/standards/technical-manuals/manual-s
- US DOE. 10 CFR 430 Subpart B, Appendix CC: Uniform test method for portable air conditioners (seasonally adjusted cooling capacity, which counts the infiltration a single-hose unit causes). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20CC%20to%20Subpart%20B%20of%20Part%20430
