Why the capacity of air conditioners is expressed in tons instead of horsepowers?

Home air conditioning, once a sure sign of plush living, has today become a way of life for practically everybody. The ton rating doesn’t refer to the weight of the equipment, but to its cooling power. A one ton unit will produce the same amount of cooling as one ton of ice melting in 24 hours.
Engineers sometimes describe cooling capacity in BTUs—British Thermal Units—per hour. One ton = 12,000 BTUs, so a one-ton air conditioning unit can remove 238,000 BTU’s of heat in 24 hours. A two-ton unit can remove twice this amount and so on. Deciding on the right size unit for your home is the easiest thing to go wrong. Many things affect the air conditioning load—the size of the house, how much wall faces the afternoon sun, the amount of shade, roof overhang, windows, even house and roof color. Air conditioning companies frequently use an estimating form that takes a number of such factors into account.

Additional reading:
Air conditioner (Wikipedia)

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