BTU (and tons of cooling)

In US HVAC, heating and cooling capacity is expressed in Btu per hour (often loosely called "BTUs"), sometimes in thousands (MBH). One kilowatt-hour equals about 3,412 Btu, which lets you compare electric and fuel-fired heating. A "ton" of cooling is 12,000 Btu/h, a unit that dates from ice-based cooling: roughly the heat absorbed by melting a ton of ice over a day.

Capacity tells you how much heat equipment can move, not how efficiently it does so; efficiency is a separate rating (COP, SEER2, HSPF2, AFUE).

Where it bites: oversizing equipment, which is common when sizing by square-footage rules of thumb, causes short cycling, poor humidity control and higher cost. Capacity should come from a load calculation.