The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment. A similar standard is the European seasonal energy efficiency ratio (ESEER).
The SEER rating of a unit is the cooling output during a typical cooling-season divided by the total electric energy input during the same period. The higher the unit's SEER rating the more energy efficient it is. In the U.S., the SEER is the ratio of cooling in British thermal unit (BTU) to the energy consumed in watt-hours.
For example, consider a 5,000-British-thermal-unit-per-hour (1,500 W) air-conditioning unit, with a SEER of 10 BTU/W·h, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).
SEER Rating and Energy Efficiency
SEER rating reflects overall system efficiency on a seasonal basis and EER reflects the system’s energy efficiency at one specific operating condition. Both ratings are useful when choosing products, but the same rating must be used for comparisons.
Substantial energy savings can be obtained from more efficient systems. For example, by upgrading from SEER 9 to SEER 13, the power consumption is reduced by 30% (equal to 1 − 9/13). It is claimed that this can result in energy savings valued at up to USD $300 per year depending on the usage rate and the cost of electricity.
US Department of Energy Guidelines
In 2011 the US Department of Energy (DOE) revised energy conservation rules to impose elevated minimum standards and regional standards for residential HVAC systems. The regional approach recognizes the differences in cost-optimization resulting from regional climate differences. For example, there is little cost benefit to having a very high SEER air conditioning unit in Maine, a state in the northeast US.
Starting January 1, 2015, split system central air conditioners installed in the Southeastern Region of the United States of America must be at least 14 SEER. The Southeastern Region includes the following states: Alabama, Arkansas, Delaware, Florida, Georgia, Hawaii, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, and Virginia. Similarly, split system central air conditioners installed in the Southwestern Region must be a minimum 14 SEER and 12.2 EER beginning on January 1, 2015. The Southwestern Region consists of the states of Arizona, California, Nevada, and New Mexico. Split system central air conditioners installed in all other states outside the Southeastern and Southwestern regions must continue to be a minimum of 13 SEER, which is the current national requirement.
Ductless Minisplit SEER
Today there are mini-split (ductless) air conditioner units available with SEER ratings up to 27.2. Traditional AC systems with ducts have maximum SEER ratings slightly below these levels. Also, practically, central systems will have an achieved energy efficiency ratio 10-20% lower than the nameplate rating due to the duct-related losses.