British Thermal Units
The cooling capacity of an air conditioner is measured in British thermal units, abbreviated as BTUs. This is a unit of heat, with one BTU equaling the warmth required to raise the temperature of a pound of water by one degree Fahrenheit. A 10,000 BTU air conditioner, then, for example, is capable of extracting 10,000 BTUs of heat from the air in an hour.
Room Size and BTUs
Bigger rooms require air conditioners with greater BTU ratings and vice versa. An average room of approximately 150 square feet is best cooled by a 5,000 BTU air conditioner, with the recommendation going up by about 1,000 BTUs for every 50 square feet greater than that. Many online BTU calculators can help you determine exactly how strong an air conditioner is needed for your specific room.
Room size is not the only variable to take into consideration when determining how powerful an air conditioner you need. Heavily shaded rooms tend to need 10 percent fewer BTUs than usual, while those receiving direct sunlight need 10 percent more. If a room is regularly occupied by more than two people, adding another 600 BTUs for each additional occupant ensures proper temperature control. The heat generated by appliances makes a kitchen require 4,000 more BTUs than another room of the same size.
Doing It Wrong
If you get an air conditioner that is too weak for your room's requirements, you will obviously be unable to attain the temperatures desired. But what many people fail to realize is that an air conditioner both cools and dehumidifies a room. Because dehumidifying takes time, a too-powerful air conditioner will short-cycle, turning itself off when the room is cold enough but leaving the air damp and uncomfortable.