First of all, capacity is the ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.
Heating and cooling systems are sized according to their tonnage. One (1) ton equals 12,000 BTU/H. Residential systems can range from 1 to 5 tons.
Sizing HVAC equipment is very important in determining your ultimate levels of comfort, cost and energy use. Never oversize! Oversized equipment increases the capital cost at the time of the installation and the costs of operating the equipment. Oversized equipment will not run as frequently or as long when it does run (known as “short cycling”), which accelerates the wear and tear on your equipment. It will also not be as energy efficient as a properly matched capacity unit. On the other hand, equipment that is undersized will have to work longer and harder to meet your comfort demands during temperature extremes. This results in lower energy efficiencies, less comfort, and the potential for premature equipment failure. In either case, both result in poor humidity control. Make sure to have a reputable, professional technician perform a home “load calculation” to determine the correct size heating and cooling system for your home.