Do you really know how close you are to Net Zero?

“The Zero Energy Performance Index (zEPI) is a straightforward scale that fundamentally shifts how we think about and measure energy efficiency in buildings,” says Jim Edelson, NBI’s Director of Codes and Policy. Edelson traveled recently to the DOE National Energy Codes Conference in Nashville to present on zEPI and explained that rather than using a “percent-better-than-code” metric, zEPI provides an easy-to-visualize scale designers and policymakers can use to track their progress to net zero. The scale, conceived by NBI Fellow Charles Eley, was adopted into the 2015 International Green Construction Code (IgCC) last fall and will be published in the IgCC when it comes out later this spring. It is an absolute scale with two established endpoints: (1) A building or policy with energy use equal to that of an average building in the year 2000 (established by the CBECS database) receives a score of 100, and (2) A building or policy with zero net energy (ZNE) use receives a score of 0. The zEPI scale extends in a linear fashion between, above, and below those two points. For example, a building that uses twice as much energy as an average building receives a score of a 200. A building that uses half as much energy as an average building receives a score of 50. But why is it necessary? Comparing the energy efficiency of buildings by referencing their “percent savings beyond code” creates confusion: “Which code?” “What year?” Given that there have been at least six model commercial energy codes available to choose from in the United States since 2000, identifying the correct baseline can take some time. zEPI sets a constant goal of zero and shifts the conversation from percent better than code to percent from zero, which is the kind of market vision that is required for buildings to achieve wide-scale net zero and ultra-low energy performance.zEPPI Scale to ZNE By setting an absolute scale, zEPI helps chart a clear course on energy policymaking to both set and track progress toward energy goals. As indicated in the graphic, Boulder, Colorado, with the country’s most stringent energy code, requires commercial building performance of 41 on the zEPI scale. This is markedly better than even ASHRAE 90.1-2013, but still just shy of the Architecture 2030 performance goal for 2010. If Boulder wanted to achieve a zero energy building code in 15 years, its City Council could set performance targets for each of the next five code cycles at 32, 24, 16, 8 and 0 on the zEPI scale. Onsite renewable energy resources would become a required building component at the lower zEPI target levels. zEPI also provides powerful feedback for understanding the energy efficiency of existing buildings. The information needed to assess the zEPI score of an existing building is the same as needed for a new building, only it will use measured data or utility bills rather than a building plan. zEPI ratings could be used to give a buyer a sense of comparative efficiency levels between buildings or as a metric for energy efficiency incentive programs. By setting a fixed, universal baseline under zEPI, energy modeling tools and energy modeling protocols will not need to be continually reengineered to adapt to each code revision. This will reduce modeling tool development costs while speeding up code implementation times. This is similar to a revision underway for ASHRAE 90.1called “Addendum bm.”Unlike ENERGY STAR scores, which measure building performance on a scale of 0 to 100 (100 being the best), zEPI begins at 100 and drives energy use down to zero. This is important because the “best” building on the ENERGY STAR scale is only as good as the best building from the year 2000, according to CBECS—not a very good energy performer by today’s standards. Since codes and many new buildings have moved significantly higher, it is difficult for the ENERGY STAR score to differentiate between a building that is a lot better or a little better than the best buildings from 15 years ago. Ultimately, zEPI brings clarity. The simple relationship between zEPI scores is readily understood by regulators, code officials, architects and engineers as well as less technically oriented building owners and tenants. With advancing energy codes and increasing attention on ZNE targets, zEPI is a tool whose time has come. For more information about zEPI including how to apply it to projects and utilize in policies, visit