Definition
Tonnage describes the cooling (and, for heat pumps, heating) capacity of a system. One ton equals 12,000 BTUs per hour, so a 3-ton AC removes about 36,000 BTUs of heat per hour. Residential systems typically range from 1.5 to 5 tons. The name is historical — it refers to the cooling effect of melting one ton of ice in 24 hours.
Why this matters for your home
Correct tonnage is everything. Too large and the unit cools the air fast but shuts off before it pulls out humidity, leaving your home cool but clammy. Too small and it never catches up in a humid Michigan July. Sizing is determined by a Manual J load calculation, not by the size of your old unit.