The term "net-zero" energy (or NZE) describes the concept of buildings generating their own energy needs by renewable on-site power generation. This is typically done by photovoltaic (PV) panels, sometimes by wind mills or a combination. There are several definitions of what "zero" means, the most common ones being net-zero "cost" or net-zero "site energy". The cost definition just means that your annual energy bill would be zero, which can be accomplished by over-producing electricity in the summer, running the meter backwards, and then using the "credit" you accumulate in the winter. Due to metering rate schedules with "time of use metering", net-zero cost can be accomplished even if you don't quite break even on actual power consumption. The higher goal is to actually offset all the kilowatts you use on an annual basis, completely eliminating the operating carbon footprint. 

Going even further, we strive towards “net-positive” energy, generating more than needed for the building operation, for instance producing extra energy for electric vehicle charging, or to offset - over time - the embodied energy it took to construct a building. 

In a competition entry we designed 240 dwelling units to exceed net-zero site energy standards, carefully modeled and calculated. In addition to offsetting all the electrical building loads, the excess capacity of the roof integrated PV systems was sized to give each household some 10,000 miles in electrical vehicle charge per year.