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VPD Calculator

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Air Conditioner Calculator

This air conditioner calculator sizes cooling for a grow room from the watts of your lights and equipment. For an ordinary room, switch to room size and it uses ENERGY STAR's chart instead.

Size it by
watts

The wall-socket watts, not the “equivalent” watts on the box.

watts

Fans, pumps, heaters and the dehumidifier: it turns all its power into heat, and is the one most people forget.

More options
people

ENERGY STAR adds 600 BTU/h for each person (in room-size mode, for each person beyond two).

Sun on the room
%

For outside air and heat through walls. 20% is our starting point.

Your unit’s EER (optional)

Energy efficiency ratio: cooling in BTU/h for each watt the unit draws, on its label or spec sheet (CEER on newer US portable units). Higher uses less electricity.

BTU/W

Shows how many watts the AC itself draws.

Cooling needed

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Heat from the equipment
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Total power in the room
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Common unit size that covers it
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Power the AC draws
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Notes

    Next: Size the dehumidifier too: an AC alone rarely keeps humidity down

    How it works

    All the electricity used in the room becomes heat there, light included, once it lands on leaves and walls. So the heat load is simply the watts in the room, converted to BTU per hour, plus a margin for heat coming in from outside.

    Getting the heat out with fresh air instead? Use the exhaust fan calculator.

    LED or HPS: which makes more heat?

    Watt for watt, the same. Every watt a light draws ends up as heat in the room. An LED makes the same light from fewer watts, so it needs less cooling for the same plants, and it sends less radiant heat down onto the canopy than an HPS bulb.

    Portable, window or mini-split?

    Check how the rating is measured. Newer US portable units show a “SACC” rating that is lower than the older figure on many boxes for the same machine. A mini-split moves the heat outside through a small pipe and needs no hose or window.

    Formula
    Heat     = (lights + other) watts × 3.412   BTU/h
Cooling  = heat × (1 + margin)
Tons     = cooling ÷ 12,000;   kW = cooling ÷ 3,412

    How to size an air conditioner for a grow room

    Most air conditioner size charts go by floor area. They are made for homes, where the heat comes from sun, walls and people. A grow room is different. Its lights and equipment turn every watt they draw into heat, far more than a home makes, so the calculator starts from the watts.

    The gap is easy to see. ENERGY STAR's chart gives a 100 sq ft room 5,000 BTU/h. Put four 600 W lights and 500 W of fans, pumps and a dehumidifier in that same 10×10 room and the equipment alone makes 9,895 BTU/h. With the 20% margin that is 11,874 BTU/h, which calls for a 12,000 BTU/h unit, more than twice the chart's answer.

    BTU air conditioner calculator: a worked example

    The calculator opens with a common tent setup: a 600 W light and 150 W of other equipment.

    • Power in the room: 750 W
    • Heat: 750 × 3.412 = 2,559 BTU/h
    • With the 20% margin: 3,071 BTU/h, or 0.26 tons, or 0.90 kW of cooling
    • Common unit that covers it: 5,000 BTU/h, the smallest size on ENERGY STAR's chart

    Two settings under More options change the answer. Each person working in the room adds 600 BTU/h. A very sunny room adds 10%, and a heavily shaded one takes off 10%. In the 10×10 example above, one person working in the room raises the need to 12,474 BTU/h, which tips it into the next size, 14,000.

    Tons, kW and EER

    US units are sold in BTU/h or tons, where 1 ton is 12,000 BTU/h. With a 20% margin, one ton covers about 2,930 W of lights and equipment. European units are rated in kW of cooling; divide BTU/h by 3,412.

    The EER on the label tells you what the unit itself draws: BTU/h ÷ EER = watts. If a 12,000 BTU/h unit is rated EER 10, it draws about 1,200 W at full power. Enter your EER under More options to see this. To see what that costs to run each month, put those watts into the electricity cost calculator.

    Air conditioner size chart for ordinary rooms

    Choose Room size for a bedroom, office or living room. ENERGY STAR's chart, for 8 ft ceilings:

    • 100–150 sq ft: 5,000 BTU/h; up to 250: 6,000; up to 300: 7,000; up to 350: 8,000
    • Up to 400 sq ft: 9,000; up to 450: 10,000; up to 550: 12,000; up to 700: 14,000
    • Up to 1,000 sq ft: 18,000; up to 1,200: 21,000; up to 1,400: 23,000; up to 1,500: 24,000
    • Up to 2,000 sq ft: 30,000; up to 2,500: 34,000

    Then adjust: take off 10% if heavily shaded, add 10% if very sunny, add 600 BTU/h for each person beyond two, and add 4,000 BTU/h for a kitchen.

    Common cooling mistakes

    • Leaving out CO₂ burners. A propane or natural gas burner adds heat and water as well as CO₂. The CO₂ calculator gives its BTU/h; add it to the margin or the other equipment.
    • Counting the lights only. Fans, pumps and the dehumidifier all turn their power into heat too.
    • Relying on fresh air in a hot room. An exhaust fan can only cool the tent to the temperature of the air coming in. When that is cool enough, the exhaust fan calculator may be all you need.
    • Sizing with no room to grow. If you plan to add a light later, enter its watts now. A unit sized right to the limit has nothing left for a hot afternoon.
    • Cooling without checking humidity. A cooler room at the same water content has a lower VPD. Check it with the VPD calculator.

    FAQ

    Questions growers ask

    What size air conditioner do I need for my grow room?

    Add up the watts of everything running, multiply by 3.412 for BTU/h, then add a margin. A 600 W light plus 150 W of fans and pumps makes 2,559 BTU/h, or 3,071 BTU/h with our 20% margin, so the smallest common room unit, 5,000 BTU/h, covers it. Each person working there adds 600 BTU/h.

    How do I size an air conditioner?

    For a grow room, by the watts in it; for an ordinary room, by floor area. ENERGY STAR's chart runs from 5,000 BTU/h for 100–150 sq ft to 34,000 for 2,000–2,500 sq ft. Take off 10% if shaded, add 10% if very sunny, 600 BTU/h per person beyond two and 4,000 for a kitchen.

    How many BTU per watt of grow light?

    3.412 BTU/h for every watt the light draws from the wall. A 600 W light adds 2,047 BTU/h and a 1,000 W light 3,412 BTU/h, before any margin. Use the wall-socket watts, not the equivalent figure on the box. The driver or ballast is part of that draw, so it is already counted.

    Do LEDs make less heat than HPS?

    Not per watt: a 600 W LED and a 600 W HPS both put 2,047 BTU/h into the room. An LED makes the same light from fewer watts, which is where the saving comes from. Every 400 W you take out of the room removes about 1,365 BTU/h from the cooling load, whichever light it is.

    How do I cool a grow tent without AC?

    With the exhaust fan, as long as the air around the tent is cooler than you want inside. Fresh air cannot cool the tent below its own temperature. Changing a 4×4 tent's air every minute takes about 142 CFM rated. If the room is too warm, cut the watts or run the lights at night.

    How many tons of air conditioning do I need?

    One ton, 12,000 BTU/h, covers about 2,930 W of lights and equipment with a 20% margin, or 3,517 W with none. A room running 2,900 W, such as four 600 W lights and 500 W of other equipment, needs 11,874 BTU/h, just under one ton. Each person working in the room adds 600 BTU/h.

    Why size an air conditioner from watts?

    Everything plugged in ends up as heat in the room: lights, fans, pumps and dehumidifiers. A NIST study of lighting found that, once things settle, the cooling load from lights equals their full electrical power. 1 watt of heat is 3.412 BTU/h.

    What does the margin cover?

    Heat this tool does not count: warm outside air coming in, sun on walls and roofs, and people working in the room. 20% is our starting point; raise it for a hot attic or a room with a sunny wall.

    Sources

    The formulas and target ranges on this page come from these references.

    1. Treado & Bean (1991). Measurement and evaluation of lighting/HVAC interaction. NIST IR 4429 (all electrical input to a light ends up in the room as heat; at equilibrium the cooling load equals the lighting power). nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir4429.pdf
    2. ENERGY STAR: Room air conditioners, sizing chart (capacity by room area; reduce 10% if heavily shaded, add 10% if very sunny, add 600 BTU/h per person beyond two, 4,000 BTU/h for a kitchen). www.energystar.gov/products/room_air_conditioners
    3. NIST Special Publication 811, appendix B.9: conversion factors (footcandle to lux, 1.076 391 × 10¹). www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9

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