This paper proposes a stable metric for comparing the energy efficiency of buildings. The paper examines the problem of using "percent savings better than "a given energy code" to appraise a building's energy performance. The baseline for this metric, "a given energy code," can vary greatly in its inherent efficiency. As an alternative, the paper recommends a new scale—the Zero Energy Performance Index (zEPI)—that can be used to evaluate buildings in a stable, elegant manner. A zEPI score of "0" would be a net zero energy building, while a score of "100" represents a building with average energy consumption as of the year 2000. The score for a given building will be higher or lower in linear relationship to the year 2000 benchmark. Finally, the benefits of zEPI are examined, including those for energy codes, building rating programs, incentive programs, and addressing non-regulated energy loads.