Estimating 20 British Thermal Units for each square foot of space, a 3,000 BTU air conditioner cools a 150 square foot or smaller space. Window air conditioners typically range from 5,000 to 24,000 BTUs.
Several factors affect what size air conditioner is needed. The height of the ceiling, the size of windows and doorways, the number of people regularly occupying the room and the amount of sun exposure come into play. A kitchen or bedroom requires more than 20 BTUs of cooling power per square foot. Choosing the right size air conditioner guarantees lower energy costs, as well as efficient cooling.