The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy offers these definitions as a guide to the emerging field of net-zero energy buildings.
• Net-zero energy buildings: A net-zero energy building produces as much energy as it uses over the course of a year. Net-zero energy buildings are very energy-efficient, with the remaining low energy needs typically met through the use of on-site renewable energy.
• Net-zero site energy: A site zero energy building produces at least as much energy as it uses in a year, when accounted for at the site. The measurement time frame is annual.
• Net-zero source energy: A source zero energy building 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 required 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 net-zero energy costs building, 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 energy emissions building produces at least as much emissions-free renewable energy as it uses from emission-producing energy sources annually. Carbon, NOx, and SOx are common emissions that zero energy buildings offset.
• Near zero energy: A near zero energy building produces at least 75% of its required energy through the use of on-site renewable energy. Off-grid buildings that use some non-renewable energy generation for backup are considered near zero energy buildings because they typically cannot export excess renewable generation to account for fossil fuel energy use.