When shopping for a window-mounted air conditioner, getting the proper size is essential. Window units are not designed to cool an entire home. They are instead used in individual rooms and offices that do not exceed a maximum of 400-500 square feet per installed unit.
The proper size to purchase begins with the room size. The room size is calculated by the length times width of the room in which the air conditioner will be installed. For example, a 10′ X 12′ bedroom would equal 120 square feet.
As a rule of thumb in Hawaii, most contractors use 50 BTU’s per square foot for calculating cooling load for un-insulated and single wall construction homes. This means that a 120 square-foot room would require approximately 6,000 BTU’s of cooling. Room size: 120 sq. ft. X 50 BTU = 6,000.