Total cooling or heating capacity

Cooling or heating capacity refers to the amount of cooling or heating that an air conditioner or heat pump system can provide in a given space.

The major factors impacting the required capacity for a room air conditioner are the size of the room, the ambient (outdoor) temperatures throughout the year, the number of doors and windows facing outside, and which way the room is facing relative to the sun throughout the year.

Capacity can be expressed in different units, the most common being Ton, kW, BTU, and HP. See separate entries for those terms in this glossary for more details.

Proper sizing is crucial for efficient air conditioning. Buying a larger capacity than is required won't make you feel more comfortable, but it will lead to inefficient performance and frequent overcooling or overheating of the room.
That said, many households don't have their necessary cooling or heating capacity calculated properly. Nevertheless, an experienced HVAC installer should calculate the required capacity to the extent that it won't overestimate or underestimate the room temperature by a large degree.