Before refrigeration air conditioning was invented, cooling was done by saving big blocks of ice. When cooling machines started to get used, they rated their capacity by the equivalent amount of ice melted in a day, which is where the term “ton” came from sizing air conditioning. A ton of cooling is now defined as delivering 12,000 BTU/hour of cooling. BTU is short for British Thermal Unit (and is a unit that the British do not use) The BTU is a unit of heating – or in this case, cooling – energy. It’s more important, however, to keep in perspective that a window air conditioner is usually less than one ton. A small home central air conditioner would be about two tons and a large one about five tons.