Defining Net Zero-Energy Buildings

nrel-report-coverHigh Performance Building projects often reference net zero-energy buildings.  The metrics used to establish goals in performance-based contracting specifications affect how buildings are designed to achieve the goal. Critically, the question becomes “How do you define net-zero energy use buildings”  The National Renewable Energy Laboratory (NREL) defines a net zero-energy building as a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies.

NREL’s Conference Paper, Zero Energy Buildings: A Critical Look at the Definition, addresses the definitions of zero-energy buildings (ZEB) and why a clear and measurable definition is needed in zero-energy projects.  Different definitions may be appropriate, depending upon project goals and the values of the design team and owner.  Four well-documented definitions are reviewed.

  • Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.
  • Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.
  • Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
  • Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources

The NREL study details design impacts of the definition used for High Performance Buildings and the large differences between the definitions.  Depending upon the goals set by the Owner, project teams will implement different project strategies, which result in significantly differing energy utilization outcomes.  Further, each goal will significantly impact whole-building project strategies to integrate efficiency measures with renewable energy supply options.

The NREL research study is a must read for all team members engaged in performance-based contracting projects.


LEED’s Value Proposition

In 2000, the U.S. Green Building Council (USGBC) introduced its Leadership in Energy and Environmental Design (LEED) green building rating system.  LEED is arguably the most popular green rating system in the United States, and has been instrumental in the progression of sustainable thinking in the built environment. Owners are asking if LEED certified buildings are directly influencing primary energy utilization and GHG emissions, and if so, how much?  Two reports attempt to answer this question based on empirical evidence.

NBI-CoverNew Buildings Institute’ report “Energy Performance of LEED for New Construction Buildings” analyzed measured energy performance for 121 LEED New Construction buildings. Measured performance results show that on average LEED buildings are saving energy.  The bad news is that there is a wide scatter among the individual results that make up the average savings, with a significant number of buildings using more energy than average for comparable existing building stock.  The take away from the NBI report is that performance energy results from LEED certified buildings vary greatly, and that better measured performance and feedback is required.


John Scofield, a professor of physics at Oberlin College, authored “No Evidence LEED Building Certification is saving Primary Energy”. Professor Scofield uses scientific data analysis of existing performances of LEED certified buildings. His report analyzes the public relations marketing versus performance data for existing LEED certified buildings.  His conclusions are, as his title suggests, there is no direct evidence linking performance of buildings to LEED certification.  Of particular interest is the collective momentum from all sides to understand the need for measuring performance of buildings.

APSImageAfter review of these reports, an argument can be made that any building that claims to be “green” or sustainable should meet the minimum requirement:

  • Disclose the building’s EUI.  If it is an existing building, disclose the existing EUI.  If it is a new building, disclose the as-designed or planned EUI.  Then, disclose the post-construction EUI based on actual performance data.
  • After this fundamental step, we can begin to debate and compare green rating systems.

Imagine in 10 years, looking back at this period and wondering how we classified any buildings as sustainable, without specifically addressing energy utilization and impacts on GHG emissions.