Cooling performance depends heavily on matching an air conditioner's rated capacity, measured in tons or BTUs, to the size and heat load of the room it serves. A unit that is undersized runs continuously without reaching the set temperature, while an oversized unit cycles on and off too quickly, a pattern known as short cycling, which leaves humidity poorly controlled even though the room feels cool. Heat load calculations factor in room dimensions, ceiling height, number of windows, sun exposure, and the number of heat-generating appliances or occupants typically present.
A general estimate uses about one ton of cooling for every 100 to 150 square feet under standard conditions, though this varies with insulation quality and climate. Airflow restrictions, such as furniture blocking vents or a dirty filter, can make even a correctly sized unit perform as if it were undersized. Ambient temperature outside also affects cooling performance, since the outdoor condenser has to reject heat into the surrounding air; on extremely hot days, cooling capacity effectively decreases.
Understanding these variables helps in diagnosing whether poor cooling is a sizing issue, an airflow issue, or a genuine mechanical fault.