The indoor coil in your air conditioner acts as a magnet for dust because it is constantly wetted during the cooling season. Dirt build-up on the indoor coil is the single most common cause of poor efficiency.
In an average air conditioned home, air conditioning consumes more than 2000 kilowatt-hours of electricity per year, causing about 3,500 pounds of carbon dioxide and 31 pounds of sulfur dioxide to be emitted at the power plant and, at average electricity prices, costs you about $150. In high-cooling climates those numbers can be doubled or even higher.
Central air conditioners use electric energy to pump heat out of your home and dump it outside. They distribute cooled air throughout your house and remove moisture from the indoor air.
The efficiency of Central A/C units is governed by U.S. law and regulated by the U.S. Department of Energy. Every A/C unit is assigned an efficiency rating known as its seasonal energy efficiency ratio (SEER). The SEER is defined as the total cooling output (in Btu-British thermal units) provided by the unit during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period.
The SEER is displayed on a yellow label affixed to the A/C unit. Higher SEERs are better. The minimum SEER allowed by law for a central A/C is 13 for a split system or 9.7 for a single-package unit. The best available SEER is about 18, while many older units have SEER ratings of 6 or less. Most consumers must consider a SEER of 13 or higher when buying a new A/C system.