When comparing central air conditioners, one term you'll see repeatedly is the seasonal energy efficiency ratio (SEER), a measure of how much energy the air conditioner uses to cool a home. Central air conditioners range from 13 to 28 SEER. The SEER is calculated by taking the total cooling output over the course of a summer, measured in British thermal units (Btu), and dividing it by the total amount of energy the air conditioner uses over that same period.
These figures are based on a theoretical average climate for the United States. In reality, of course, the same air conditioner's performance may vary considerably based on how hot and humid it is outdoors. The American Council for an Energy-Efficient Economy (ACEEE) recommends that people who live in hot and humid climates choose an air conditioner with a SEER of at least 15.
Another measure of air conditioner efficiency is the energy efficiency ratio (EER). This measures the air conditioner's efficiency at any given moment. It's simply the cooling capacity of the air conditioner, as measured in Btu per hour, divided by its energy consumption in watts. The ACEEE says EER is more important than SEER for those living in hot and dry climates, since it measures how well the unit will perform on the hottest days.
Energy Star ratings for central air conditioners are based on both SEER and EER. To qualify for the label, a typical split-system air conditioner must have a SEER of at least 14.5 and an EER of at least 12. For single-package units, the requirements are lower: 14 SEER and 11 EER. If you buy any central air conditioner that meets these guidelines before the end of 2013, you are eligible for a $300 federal income tax credit.
The Energy Star label is only one award a central air conditioner can earn for efficiency. The Consortium for Energy Efficiency (CEE) has defined two additional tiers of energy efficiency beyond Energy Star. CEE Tier-2 air conditioners have a minimum of 15 SEER and 12.5 EER; Tier 3 starts at 16 SEER and 13 EER. State and local governments, as well as utility companies, may offer rebates for choosing a central air conditioner that meets one of these higher standards.
In New Jersey, for example, people affected by Superstorm Sandy in 2012 can get a $500 rebate on Tier-3 equipment as part of the state's hurricane relief program. Depending on what rebates are available in your area, you may be able to pay less out of pocket for a high-efficiency central air conditioner than you would for a less efficient model.