Tonnage or Capacity
The tonnage of an air conditioner is the rating of a unit's ability to remove heat from your home or business. A ton of cooling is 12,000 BTU’s and residential air conditioners are made in sizes of 1.5 ton to 5 ton in half ton increments. A BTU is a measure of heat which stands for British Thermal Unit. One BTU is equal to the amount of heat one wooden match puts out.
In other words, a 1.5 ton air conditioner will remove the amount of heat the 18,000 matches would emit if they were all burning at the same time. The reason it is called a ton is that is the amount of heat (12,000 BTU) it would take to melt one block of ice weighing one ton in one hour. The job of an air conditioning contractor is determining the correct size, or tonnage of a unit for you, and in this case it is not a matter of "the more the better."
An oversize air conditioner will not remove the proper amount of moisture from the air to give you the comfort you deserve. An undersized air conditioner will run constantly even at moderate temperature and will not adequately condition the space. There are some air conditioners you can slightly oversize and still get very good dehumidification and those are two speed and variable speed units. A variable speed furnace will also help in having a system that not only keeps you cool, but dry also.