By: Daniel Overbey The term ‘net-zero energy’ is ubiquitous within the high-performance community today. But what does it actually mean? In practice, use of the term ‘net-zero energy’ varies from project to project and it can be used to reference quantities of kilowatt-hours, energy costs, or emissions. Moreover, it can reference the energy balance at the site scale or consider the energy expended to deliver electricity from a remote off-site source to a building. The National Renewable Energy Laboratory (NREL), a prime authority on the topic, generally considers four basic definitions of net-zero energy, as defined in their report Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options. Net-Zero Site Energy In its most common definition, net-zero energy can simply imply that a building generates (at least) as much energy on-site as it consumes on-site. This doesn’t happen all at once. At certain times of the year a building will consume more than it produces and vice versa. But measured over the course of 365 consecutive days, if the building produces as much (or more) energy on-site as it consumes, then it achieve net-zero energy. NREL specially calls this net-zero site energy because the definition only considers the energy balance at the site scale. Net-Zero Source Energy What if we expand the definition of net-zero site energy to instead consider all of the energy expended to both generate and deliver the energy to the site from a remote point of generation (such as a power plant)? The transmission losses and other inefficiencies embodied in the delivery of utility-scale energy generation are tremendous. According to the U.S. Department of Energy, over 60% of the energy expended to generate and deliver electricity to a site is lost to these various inefficiencies. If energy was produced on-site, these very real quantities of energy would not be wasted. Therefore, NREL uses the term net-zero source energy to expand the net-zero term to consider the total amount of energy expended to generate energy for and deliver it to a building, as well as the quantity of energy consumed by the building. Net-Zero Energy Cost Not considered in either of the previous definitions is the cost equation. Utilities generally charge more for energy than they pay for it. As a result, a net-zero site energy building may not equate into a zero balance in terms of energy costs. For this reason, NREL uses the term net-zero energy cost to describe a building that sells more power to a utility than it purchases. Net-Zero Emissions Also missing from the previous definitions is the balance of greenhouse gas emissions. NREL uses the term net-zero emissions to describe a building that generates (at least) as much renewable energy as it consumes from non-renewable sources. This energy can be produced either on-site or purchased. In other words, buying renewable energy credits (RECs) to offset non-renewable energy consumption does count in this definition. For additional information on net-zero energy building design, please consider this VoiceThread presentation from BDMD Architects entitled Efficiency: Toward Net-Zero Energy.