The amount of heat an air conditioner can remove from a room is measured in BTUs, or British Thermal Units—larger rooms require a unit with a higher BTU. Energy Star recommends units around 6,000 BTU for rooms up to 250 square feet; 8,000 BTU units up to 350 square feet; 10,000 BTU units up to 450 square feet; and 12,000 BTU units up to 550 square feet.