The Home Energy Rating System (HERS) Index was established by the Residential Energy Services Network (RESNET), a California-based nonprofit, in 2006. In simple terms, the HERS Index compares a new home to a “reference home” of identical size that meets the minimum requirements of the 2006 International Energy Conservation Code (IECC).
To calculate a home’s HERS Index, a rater inputs data about the house into one of four RESNET-accredited software programs. The reference home built to the minimum 2006 IECC requirements gets a score of 100 (it use to be 80, so do not let that confuse you).
Using the HERS Index, a lower score is better. The index correlates with percentage improvements versus the reference home and scores can range from 0 to 150. For instance, an EPA Energy Star home must be no higher than 85. A score of 85 means that the home uses 15% less energy than the standard reference home. A theoretical net-zero home would score 0 on the HERS Index. According to RESNET, a typical resale home scores 130.
While the HERS Index has existed for several years, it has recently gained considerable traction. RESNET has recently entered into an agreement with 20 of the country’s largest home builders to have their homes rated by HERS. Adding legitimacy, several other green home rating systems reference the HERS Index in some manner, including EPA’s Energy Star program and USGBC’s LEED for Homes rating system. Also, the EnergySmart Home Scale (E-Scale) used by the DOE Builders Challenge is based on the HERS Index.
It appears that the HERS Index is quickly becoming the building industry’s standard for evaluating the relative energy performance of homes.