What does net zero energy really mean?
For a grid-tied net-zero-energy building, this is about offsetting the energy consumed with energy produced by renewable, emissions-free means. Why not offset by purchasing renewable energy or just buying carbon offsets? This could be called a Net-Zero Off-Site Energy Building. While there is no standard or third-party verification system for net zero energy, the four most common definitions do not allow offsets for off-site renewable energy. A 2006 paper (PDF) by the National Renewable Energy Laboratory explains these definitions in detail.
- 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.
A building constructed to Passive House standards can be a Net Zero Energy Emissions building by adding a photovoltaic array or a wind turbine. The Passive House Planning Package (PHPP) software provides a tool to calculate what is required for carbon neutrality. All other things being equal, the Passive House building will cost less to operate.
See here for a more technical post on this topic.
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