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Despite sharing the name 'zero energy building', there are several definitions of what this means in practice, with a particular difference in usage between North America and the rest of the world. Net zero cost At the simplest level, 'net zero energy' relates to the price of energy. In such a building, the cost of purchasing energy is balanced by income from sales of electricity to the grid of electricity generated on-site. Whether this balance can be maintained over the medium to long term is subject to fluctuations in energy prices. To distinguish it from other types of zero energy building, the term net zero energy cost building may be used. For commercial buildings, a cost ZEB is typically the hardest to reach, and is very dependent on how a utility credits net electricity generation and the utility rate structure the building uses (Torcellini et al. 2006). Net zero site energy use In this type of zero-energy building, the amount of energy provided by on-site renewable energy sources is equal to the amount of energy used by the building. Net off-site zero energy use This variation of zero-energy building considers that purchasing energy used by the building from 100% renewable energy sources, even if the energy is generated off the site, is sufficient to constitute a zero-energy building. Net zero primary energy use More sophisticated than the previous definitions, the zero primary energy building or zero energy source building recognises that the off-site generation of energy, particularly electricity, is very inefficient. Typically only around 35% of the energy used in a conventional fossil fuel power plant is converted to electricity, with the remainder lost as waste heat. Further losses accumulate during electricity transmission. Because of this, in order to meet the definition of zero primary energy use, the amount of electricity exported must be substantially higher than the amount of energy registered on the electricity meter. Net zero energy emissions Outside the United States and Canada, a net zero energy building is generally defined as one with zero net energy emissions, also known as a zero carbon building or zero emissions building. Under this definition the carbon emissions generated from on-site or off-site fossil fuel use are balanced by the amount of on-site renewable energy production. A more honest definition A more honest definition includes not only the carbon emissions generated by the building in use, but also those generated in the construction of the building and the embodied energy of the structure. Others debate whether the carbon emissions of commuting to and from the building should also be included in the calculation.
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