A classification of building energy performance indices

Energy performance indices are used around the world to evaluate and monitor residential and commercial building energy performance during design, construction, renovation, and operation. The two most common indices are Asset Ratings and Operational Ratings. Asset Ratings are based on modeled energy use with uniform conditions of climate, schedules, plug loads, occupancy, and energy management. Operational Ratings are based on measured energy use, often normalized for relevant variables like climate and level of energy service. Surprisingly, there is almost no discussion in the literature about the technical basis of these ratings and what they are attempting to measure. This paper analyzes the merits and weaknesses of the common ratings and introduces additional energy performance indices, in particular the Operation and Maintenance (O&M) Index, which is the ratio of the energy consumption as measured at the meter to the simulated energy performance, calibrated for the actual operating conditions of the building. We provide examples of how such indices are currently used, although we do so as examples to illustrate our hypothesis as to what indices are most helpful to improve energy management, rather than as a comprehensive review. We show how these indices work together to provide better feedback to energy managers. The Operational Rating answers the question: "how does the energy intensity of this building compare to its peers?" The Asset Rating answers the question: "how efficient is this building?" The O&M Index answers the question: "how well is this building being managed?" These questions are useful to answer in the context of a comprehensive energy management program, such as would be required by an Energy Management System standard.