Accuracy of Home Energy Rating Systems

Actual residential energy bills were compared to estimated energy use and energy costs for four Home Energy Rating Systems (HERS) including the CHEERS Version I rating system. CHEERS tended to overestimate the actual energy cost by approximately 50%, with large variation in the accuracy of individual ratings. In addition to normal variations in occupant behavior, possible sources of error include inadequate rater training and incorrect assumptions about average occupant preferences for thermostat setpoints. However, for houses built between 1990 and 1994 CHEERS' average error was relatively small. The other three HERS all exhibited relatively small average errors in the estimated energy cost or energy use. Contrary to expectations, none of the HERS showed any clear relationship between the rating score and actual energy cost. Furthermore, all of the case studies tended to support the hypothesis that occupants of more energy efficient houses "takeback" some of their energy savings by using more energy services. This makes it difficult to use standard occupant behavior assumptions for all house types and still expect average error to be small for all types of houses. The case studies also demonstrated that it is more difficult to accurately predict energy use in mild climates than in more severe climates. While accuracy does not appear to be a major concern of the HERS industry or lenders who participate in HERS programs, it is still important, especially for consumers who make investments based on HERS ratings. Improvements in accuracy can be made with additional research and possibly through some fundamental modifications in the rating systems.