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What size AC unit do I need for my garden?

Air conditioning is one of the most important tools when growing indoors, particularly during the hot summer months. Using an air conditioner provides your garden with an accurate way to offer temperature control, and can also substantially lower humidity levels as an added bonus. The investment into an air conditioning system will remove many of the hassles that afflict indoor gardens.

AC systems are based on a unit of measurement called tons, which is used to describe the unit’s cooling capacity. Determining the required tonnage will help you understand which unit you’ll need for your garden. One ton of refrigeration is equivalent to 12,000 BTU’s (British thermal unit), which is a measurement of how much energy it takes to raise or lower 1 pound of water by 1°F. Raising the temperature requires increasing the heat energy into a space. Lowering the temperature means withdrawing or removing the heat from that space. These temperature changes require energy in the form of electricity – or in this case, BTUs, to accomplish that task.

To determine the BTU requirements for your room, you first need to know the size of the room and the heat load that it carries before adding any equipment. Many people base their final calculation simply off the number of lights, however the room size, presence of ballasts, dehumidifiers, and other equipment will also have a significant effect on which unit to use. Factors like high ceilings, poor insulation, and excessively high ambient temperatures can also drastically affect the BTU requirements. Use the sizing chart below from Hydro Innovations to establish your approximate size requirements.

Suggesting Cooling BTU’s Before Adding Equipment 

Room Dimensions (Square Feet)

Recommended A/C BTU

5’ x 5’ (25 sq. ft.) 2,500
5’ x 10’ (50 sq. ft.) 3,100
10’ x 10’ (100 sq. ft.) 4,200
10’ x 15’ (150 sq. ft.) 5,300
10’ x 20’ (200 sq. ft.) 6,500
10’ x 25’ (250 sq. ft.) 7,500
15’ x 20’ (300 sq. ft.) 8,700
15’ x 25’ (375 sq. ft.) 10,400
20’ x 20’ (400 sq. ft.) 11,000
20’ x 25’ (500 sq. ft.) 13,100
20’ x 30’ (600 sq. ft.) 15,400
30’ x 30’ (900 sq. ft.) 22,000

Next, add 4,000 BTU’s for each 1,000 watt light bulb present in the grow room. Therefore if you need to cool 3,000 watts, you will need an additional 12,000 BTU’s. For a grow setup with two 600-watt bulbs (1,200 watts), you would require slightly over 4,000 BTU’s, or approximately 4,800 BTU’s. For each magnetic ballast present in the grow room add another 3,500 BTU’s, and for digital ballasts, add 2,500 BTU’s. If you are using a dehumidifier, you will also need to determine how many pints per day it is rated for, then multiply that number by 30 BTU’s.

We generally recommend oversizing your air conditioning system by 5-10%, as it’s better to slightly overrate how many BTUs you need, rather than to not have enough. However keep in mind that larger units can quickly become inefficient in their electricity use and take longer to remove heat, therefore bigger is not always better. The closer the unit is sized to the actual requirements, the more efficient it will be, and the lower your monthly electricity bill.

With smaller setups a window unit or portable air conditioner will be sufficient, but for mid-size to large setups a more sufficient form of air conditioning equipment should be used, particularly in setups with over 4,000 watts. For larger units, we recommend using the XL series from Excel Air Systems. This system is a complete package, making it the perfect solution for any grow room. Due to its plug and play design, you can install the system without needing a certified installer. This air-cooled A/C unit is pre-charged with refrigerant and uses re-sealable fittings for quick and efficient connects and disconnects. Air Conditioning has never been so effortless.

For an easier method of calculating your exact requirements, we recommend using this helpful chart from Hydro Innovations. To use, simply fill in the blanks on the left, multiply them by the BTU’s indicated, then add them up to determine your total heat load.

6 thoughts on “What size AC unit do I need for my garden?

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  • I have some questions about your BTU math for an AC unit in a grow room. First, if a 1000w light needs 4000 btu, why would a digital ballast need 2400? The heat from the ballast is not 60% of a 1000w light. I would find that hard to believe. Second, I run a 400 sqft room with a 7ft ceiling. According to your math, I would need over 82,000 btu to cool the room. That means I’d need to use a 7 ton unit??? I find that hard to believe. Have you put these numbers to the tests?

  • These BTU approximations are recommendations under extreme conditions. Pairing an air conditioner with an indoor grow room is not always as simple as one might think. Here at San Diego Hydroponics we help customers who live anywhere from beach and coastal communities to high mountain and desert communities as well. Air conditioning requirements will vary greatly throughout different seasons of the year also. This chart you referenced on our website is listing extreme conditions. We do not take credit for this chart, and we cite and credit for their information and research. I would reference this chart if you live in a community with regularly high temperatures in the mid to high 80s throughout the year; and also if you would like to utilize parabolic hoods and other non-air-cooled forms of lighting. During periods of high temperature, when the ambient temperatures outside the indoor garden greatly exceed comfortable levels in the indoor garden, large amounts of air conditioning are required. I apologize for any confusion and would be more than happy to correspond with you to further help you pair the amount of air conditioning required to cool your indoor garden. I hope this helps and if you have any further questions feel free to give us a call anytime or shoot me another email at your convenience. Happy Gardening

    Your friends at San Diego Hydroponics and Organics

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