Today's best air conditioners use 30% to 50% less energy to produce the same amount of cooling as air conditioners made in the mid 1970s. Even if your air conditioner is only 10 years old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model.

Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the "ton," which is 12,000 Btu per hour.

How big should your air conditioner be? The size of an air conditioner depends on:

• How large your home is and how many windows it has;
• How much shade is on your home's windows, walls, and roof;
• How much insulation is in your home's ceiling and walls;
• How much air leaks into your home from the outside;
• How much heat the occupants and appliances in your home generate.

An air conditioner's efficiency, performance, durability, and initial cost depend on matching its size to the above factors.

Make sure you buy the correct size of air conditioner. Two groups —- the Air Conditioning Contractors of America (ACCA) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) —- publish calculation procedures for sizing central air conditioners. Reputable air conditioning contractors will use one of these procedures, often performed with the aid of a computer, to size your new central air conditioner.

Be aware that a large air conditioner will not provide the best cooling. Buying an oversized air conditioner penalizes you in the following ways:

• It costs more to buy a larger air conditioner than you need.
• The larger-than-necessary air conditioner cycles on and off more frequently, reducing its efficiency.
• Frequent cycling makes indoor temperatures fluctuate more and results in a less comfortable environment.
• Frequent cycling also inhibits moisture removal. In humid climates, removing moisture is essential for acceptable comfort.
• In addition, this cycling wears out the compressor and electrical parts more rapidly.
• A larger air conditioner uses more electricity and creates added demands on electrical generation and delivery systems.