There are several different definitions that are used for net zero energy buildings. Late in 2015, the US Department of Energy came out with a definition to help create a uniform language. The DOE definition states: “An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.” While this definition will promote energy use reductions, it unfortunately, falls short of creating buildings that will actually get us to carbon neutrality. The issue with this definition is that it allows for carbon emitting combustion (fossil fuels) to still be used in buildings, such as natural gas boilers for hot water or cooking equipment. So buildings built to this definition can be net zero energy because they generate renewable energy to offset all of the energy, both electric and combustion, that they burn, but they are not carbon neutral and that is what we need if we intend to prevent global disaster.
The City of Cambridge passed a Net Zero Emissions Cambridge policy in 2015. This city policy defines net zero buildings as,
“An annual balance of zero greenhouse gas emissions from building operations achieved on a building-by-building basis using energy efficiency, renewable energy, and if necessary carbon offsets or, potentially, credits as a temporary measure.”
This definition goes beyond the DOE “Net Zero Energy” definition in that it requires there be no carbon emitting combustion on site within 10 years. While a project could potentially use combustion as a temporary measure, it does not make sense to install a combustion system only to replace it in 10 years with an electric system. The Cambridge policy is truly forward thinking to achieve carbon neutrality.