What Is a Ton?

Not the unit of weight, but the measurement of BTU energy?  We have been talking a lot about air conditioning in the heat of the summer, and I (just this week), added another AC unit to the shop.

But, the conversation wove around to the unit of measurement that measures the cooling capacity of an air conditioning unit, and why do they call it a tom?  Just a little research and we find that it's an archaic term that is based on the amount of energy needed to melt a ton of ice.

A ton is the cooling capacity of an air conditioning system. One ton is equal to the amount of heat required (288,000 Btu) to melt one ton of ice in a 24-hour period. A one-ton air conditioner is rated at 12,000 Btu per hour (288,000/24). A two-ton unit would be rated at 24,000 Btu per hour. Typical residential central heating systems provide up to 5-tons of cooling. Commercial systems can range anywhere from small, three-ton rooftop units, to 1,500-ton chiller systems. Why a ton of ice? The term is leftover from the days before mechanical cooling when ice was an essential part of refrigeration.

So, if a ton is equal to 12,000 BTU/hr, then I have 4.5 tons of cooling hanging in the wall of my shop. (Three 18K units) 

So, now I know what a ton of AC capacity is:  Basically, 12,000 BTU.