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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. Whether choosing central air conditioning or a room air conditioner, here's what you need to know about picking the right unit for your needs:

Choosing the right size

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.

Why bigger isn't better

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.

Air Conditioner Efficiency

Each air conditioner has an energy-efficiency rating that lists how many Btu per hour are removed for each watt of power it draws. For room air conditioners, this efficiency rating is the Energy Efficiency Ratio, or EER. For central air conditioners, it is the Seasonal Energy Efficiency Ratio, or SEER. These ratings are posted on an Energy Guide Label, which must be conspicuously attached to all new air conditioners. Many air conditioner manufacturers are participants in the voluntary Energy Star labeling program. Energy Star-labeled appliances mean that they have high EER and SEER ratings. (You can learn more on the Energy Star Web site.)

In general, new air conditioners with higher EERs or SEERs sport higher price tags. However, the higher initial cost of an energy-efficient model will be repaid to you several times during its life span. Your utility company may encourage the purchase of a more efficient air conditioner by rebating some or all of the price difference.

Buy the most efficient air conditioner you can afford, especially if you use (or think you will use) an air conditioner frequently and/or if your electricity rates are high.

Room Air Conditioners -- EER

Room air conditioners generally range from 5,500 Btu per hour to 14,000 Btu per hour. National appliance standards require room air conditioners built after January 1, 1990, to have an EER of 8.0 or greater. Select a room air conditioner with an EER of at least 9.0 if you live in a mild climate. If you live in a hot climate, select one with an EER over 10.

The Association of Home Appliance Manufacturers reports that the average EER of room air conditioners rose 47% from 1972 to 1991. If you own a 1970s-vintage room air conditioner with an EER of 5 and you replace it with a new one with an EER of 10, you will cut your air conditioning energy costs in half.

Central Air Conditioners -- SEER

National minimum standards for central air conditioners require a SEER of 9.7 and 10.0, for single-package and split-systems, respectively. You do not need to settle for the minimum standard -- there is a wide selection of units with SEERs reaching nearly 17.Before 1979, the SEERs of central air conditioners ranged from 4.5 to 8.0. Replacing a 1970s-era central air conditioner with a SEER of 6 with a new unit having a SEER of 12 will cut your air conditioning costs in half.