US-DOE just released a document called: A Common Definition for Zero Energy Buildings”The question is, is it useful?
The actual definition for a ZEB building proposed in the document, besides many explanations and other scale level definitions, is:
a ZEB building is 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.
Some thoughts with this building level definition:
1 Source energy: the formulation does not refer to local use, but to import and export: Imported can be coal for instance, produced can be PV: But then this is calculated in primary energy: PV produced in the building as final energy, has to produce about twice more to to compensate for the ineffective conversion of coal while it could have been used directly…
2 In this definition there is in fact no coupling with a building: you could sell all Solar energy, regardless the house performance : There is no ZEB building, there is just a profit making use of the building, exploiting the roof for instance.
3 It refers to a “energy efficient building”? Why would that be a requirement? 0 is 0 , that can be at different combinations of demand and matching levels of production. The optimum between the two should be decided by lowest materials impacts. To avoid shifting the burden from energy to materials. And that can be at a more or less energy demanding building. (see ref: 3)
In other words: I think this is a not so strong definition: it is formulated in a way that many options remain open and even fossil fuels can remain in business .
For more on this issue see
my blog on EPBD (http://www.sustainablebuilding.info/?page_id=328) ,
a discussion document on ZEB definition https://www.researchgate.net/publication/282210899_0-energy_or_Carbon_neutral_Systems_and_Definitions-discussion_paper , and
the paper on A paradigm shift from 0-energy and beyond: http://www.mdpi.com/2075-5309/5/1/1/htm.
the DOE report can be downloaded here: http://energy.gov/eere/buildings/downloads/common-definition-zero-energy-buildings