This depends on your room size. An aircon's power is measured in BTUs (British Thermal Units). This describes how much heat it takes to raise the temperature of one pound of water by 1°F. So, the higher the BTU, the larger the room an air conditioner can cool. We recommend a small room (18 sq/m) uses a 7,000 BTU air conditioner. Medium-sized rooms (24 sq/m) should use a 10,000 BTU air conditioner. And large rooms (36 sq/m) should look for a more powerful unit such as a 18,000 BTU air conditioner. Top tip: To cool conservatories you need a more powerful unit than a room of the same size with small windows.