A 5,000 BTU air conditioner cools 100 to 150 square feet. The amount of sunlight, the number of people that regularly occupy the room and the purpose of the room affect the efficiency of an air conditioner.
For maximum air conditioner efficiency, decrease BTU capacity by 10 percent in heavily shaded rooms, and increase it by 10 percent in sunny rooms. If more than two people are in the room for extended periods, increase BTU capacity by 600 for each additional person. Increase the capacity of a unit in a kitchen by 4,000 BTU to manage the heat produced by cooking and appliances.