How to Choose the Right Size Room Air Conditioning Unit

Choosing the right capacity individual room air conditioning is critical to comfort. Getting it wrong can be unnecessarily inefficient and wasteful. An AC unit that is too small will overwork while never adequately cooling the space in question. It will also use more energy, as it will tend to run continually. A unit that is too large can result in uneven cooling and frequent starts and stops, creating the most energy-intensive conditions. (The startup sequence is where a great deal of the energy is consumed.) Cooling air too fast does not allow air farther from the unit to properly mix and smoothly come up to an even temperature. The thermostat will tend to shut off the unit too quickly while leaving the air in the farther reaches of the room still too hot. Not only that, it will just plain cost more, with no advantage to you during use.


Choose the Right Room AC Unit Rating

If you are going for a room air conditioning unit, choosing it will be easier than for a central AC unit. Much of your work will be done by the rating assigned to it by the manufacturer, coupled with your own simple calculations as to area of the space and some other conditions described below:

  • Insulation of the building to be installed in: If you know the building is un-insulated or poorly insulated, you may want to adjust upwards in your thinking on the capacity of the unit you purchase. In very hot climates room temperatures can be dramatically affected by lack of sufficient insulation.

  • Part of the building to be cooled: If you are intending on using the new unit in a central Room or one with a northern exposure, or are sandwiched between floors in a larger structure, this may affect the overall temperature your AC unit will be expected to counteract. If you are in a place where other rooms are also being cooled by space AC units, you may not need as hefty a unit, as some of the work will be done by adjoining rooms being cooler. However, also realize that if you are not the person operating these, those other units may not be running on the same schedule as yours and you cannot depend on the added cooling they might provide.

A rule of thumb is to add 10% for rooms exposed to direct sunlight conditions or subtract 10% for shaded conditions.

  • Cooling kitchens or spaces with heat-intensive equipment operating in them: Rule of thumb: add at least 4000 BTUs to the final equation.

  • Number of people normally expected to occupy the room: The general rule here is, if more than two people, add 600 BTUs per person.

Calculating Proper Capacity Room Air Conditioning

This part is relatively simple. Measure the length and width of the room. Multiply the two by each other for the total area. If it is an irregular shape, break it down as best you can into smaller regular shapes, calculate their areas and get the total of these that add up to the whole area of the room. Triangular areas can be calculated by multiplying the length x width x 0.5.


Capacity of Air Conditioners is given in (BTUs) per hour. The US Department of Energy posts these guidelines for BTUs per square footage on their Energy Star website:

  • 100 up to 300 sq. feet: 5,000 to 7,000 BTU

  • 300 up to 550 sq. feet: 8,000 to 12,000 BTU

  • 550 up to 1,000 sq. feet: 14,000 to 18,000 BTU

  • 1,000 up to 1,200 sq. feet: 21,000 to 24,000 BTU

  • 1,500 up to 2,000 sq. feet: 30,000 BTU

  • 2,000 up to 2,500 sq. feet: 34,000 BTU

The savvy homeowner ready to make a larger commitment may want to step up to a central AC installation. This can be more efficient and can be combined with a furnace or heat pump installation during an upgrade to a new central heat system. The combination can save operating expenses as well as streamline installation and make the best use of ducting and venting issues. There are also rebates and incentive programs through government and private industry to help cover such upgrades. Your local professional HVAC representative can assist you with making the right choice, be it with the purchase of a room air conditioning unit or with central AC installation.

Creative Commons Attribution: Permission is granted to repost this article in its entirety with credit to Alley and Co. Heating/Air and a clickable link back to this page.

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