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The efficiency of central air conditioning systems is rated by a Seasonal Energy Efficiency Ratio (SEER). SEER ratings typically range from 13 to 23, with the highest numbers indicating the most efficient units that offer the most energy savings year after year. Fortunately, great strides have been made in the last 10 years to increase the efficiency of new air conditioners and heat pumps.

The typical SEER rating of air conditioners manufactured before 1992 is about 6.0. In 1992, the federal government established the minimum cooling efficiency standard for units installed in new homes at 10. The minimum SEER value changed again in 2006 to a SEER of 13. The SEER rating is usually shown on a yellow and black EnergyGuide sticker attached to the outside unit of the air conditioner.

Central air conditioners that are in the top 25 percent of efficient models may carry the Energy Star® label. To qualify, they must have a minimum SEER efficiency level of 14.5. Additionally, Energy Star® models must also have a minimum Energy Efficiency Ratio (EER) of at least 12 for split systems, and of at least 11.0 for single-package models. Air conditioners that bear the Energy Star® label may be twice as efficient as some existing systems.

Heat pumps also have heating efficiency ratings, indicated as a Heating Seasonal Performance Factor (HSPF). In general, the higher the HSPF rating, the less electricity the unit will use to do its job.

In 1992, the federal government established the minimum heating efficiency standard for new heat pumps at 6.8 HSPF. (Most heat pumps manufactured before 1992 had HSPF ratings below 5.) The minimum HSPF standard changed again in 2006 to an HSPF of 7.7. High-efficiency central air-conditioning heat pumps can also qualify as Energy Star® models.

Higher efficiency units usually cost more to purchase initially, but save money in the long run on operating costs.