The British thermal unit rating is a measurement of how much heat an air conditioner can remove from a room and is important in determining what size unit is needed. The ratio of Btu per hour to power input wattage determines the energy efficiency ratio of a unit.
Btu ratings of window air conditioners generally range between 5,000 and 20,000, as of 2015. The amount of Btus needed to cool a room is dependant on the square footage of that room as determined by multiplying the length of the room by the width. In general, a 5,000-Btu unit cools a 150-square-foot room, and a 24,500-Btu capacity machine cools a 1,560-square-foot area.
In addition to room size, the location, occupancy and use of the room are also factored into the Btu rating an air conditioner needs to cool the room. Rooms in sunny locations need 10 percent more Btus, and rooms in shady areas need 10 percent less. Kitchens require an additional 4,000 Btus, and each person occupying the room adds 600 Btus to the equation.
After the Btu requirements are established, the energy efficiency of the specific air conditioner unit is examined using the energy efficiency ratio ratings. As of 2015, newly manufactured units range from 8 to 11.5 EER, with the higher numbers being the most efficient.