Your comfort while using air conditioning depends both on reducing air temperature and removing humidity. An air conditioner should ideally run for 20 minutes or more on each cycle to cool the indoor air and to reduce humidity.
Oversized air conditioners run in short inefficient cycles. They waste energy since they must run for a few minutes at the beginning of every cycle just to cool down their coils, fins, and ducts. Only then can they cool and de-humidify your home. That original cool-down energy is lost after the air conditioner shuts off.
An air conditioner’s ability to remove heat is expressed in Btus per hour, or “tons” of cooling. Each ton equals 12,000 Btus per hour. Your air conditioner should have a ton of cooling capacity for every 600 to 1,200 square feet of floor area, depending on your home’s energy efficiency and your local climate.
A poorly shaded home with little insulation and lots of air leaks might need a ton of air conditioning for every 600 square feet of floor area. A well-insulated and well-shaded home with few air leaks might only need one ton per 1200 square feet.
Reputable contractors size air-conditioning systems, using computer programs or manual calculations. To ensure correct sizing, ask your contractor to show you the finished calculations for your home. Then ask the contractor to install the smallest size air conditioner capable of cooling your home.
You should also ask about the moisture removal capacity of the system and its sensible heat factor (SHF). The SHF is an important sizing consideration that rates the air conditioner’s moisture-removal capacity. The SHF is a decimal number between 0.5 and 1.0. The lower the SHF, the less efficient the system will be at cooling, but the more moisture it will remove from the air. Homeowners in dry and moderate climates prefer air conditioners with a high SHF, because they are more efficient. Homeowners in humid climates want a lower SHF to reduce humidity for better comfort.
For more information on air conditioning see The Homeowners Handbook to Energy Efficiency , Your Home Cooling Energy Guide, Chapter 8 of Residential Energy: Cost Savings and Comfort for Existing Buildings or for specific information about servicing air conditioners for energy efficiency, see Saturn Hydronic Systems Field Guide.