Tonnage and BTUs (British Thermal Units): Central air conditioners are sized in tons and BTUs (1 ton = 12,000 BTUs). The higher the tonnage and BTUs (ranging from 1 ton/12,000 BTUs to 5 tons/60,000 BTUs), the more powerful the unit. What size BTU/tonnage you need depends on the size of your home and a lot of other factors. A professional will need to perform what's called a "load calculation" to find the correct size.