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.