A solar-electric-system generates electric energy for the house and over the year the kilowatt-hours of electric energy exported to the grid equals the kilowatt-hours of electric energy imported from the grid. A NZE house is always connected to the electric grid. In the winter more electricity is imported than exported and in the summer more is exported than imported -- the net result over the year should be roughly zero kilowatt hours.

Since the roof size available is limited a NZE house must naturally reduce its energy consumption dramatically so that the size of the solar-electric system is practical. This always starts with exceptional insulation (in the range of R50 or higher), triple-glazed windows, and significant attention to all aspects of the building envelope.

A NZE house also must have a method of collecting heat in the winter. Heat gain methods can include large windows for passive solar space heating, solar thermal collectors for domestic hot water, or other solar thermal air-heating technologies. A NZE house always starts as an ultra-efficient house so that the size of the solar-electric system can be fit onto the available roof space.

NZE houses still pay for electricity utilities. Electric service and the delivery of electric energy will still be present even though the net kilowatt-hours over the year are zero. A NZE house will not typically use natural gas because of the large fees for connecting to the gas grid. Some NZE houses choose to include a wood stove for comfort and additional energy security however a wood stove is "input energy" that is not included in the NZE calculations. A house that uses external energy from wood, natural gas, or propane is only a NZE house if the solar-electric system exports to the electric grid an amount of energy equal to this external energy source.

A NZE house would have an Energuide rating of 100 by definition (essentially consuming no net outside energy to operate the house).

The most common NZER type of house is an ultra-efficient house that would become NZE with the addition of a suitably-sized solar-electric system. Often an NZER house will have the insulation and passive solar features included and be pre-wired for a solar system. The cost of the solar-electric system is left until later when budgets permit.

A true NZER house is one where the required solar-electric system to achieve the NZE goal is of a size that will practically fit on the house. Occasionally we see incorrect claims of NZER where the required solar-electric system would be impractically large.

NZER houses usually start with an Energuide rating above 86.

In our definition a NNZ house needs to be quite close to NZE status to qualify as NNZ and almost always will have solar electric and/or solar thermal systems. In our definitions a NNZ house would have an EnerGuide rating of at least 90.

In our definitions an EE house is usually one where an external source of energy is used for heat. In order to take an EE house to a NNZ or NZE status you would need to add enough solar collectors (electric or thermal) to offset most (NNZ) or all (NZE) of the annual external energy consumed.

We may list a home as SC if an Energuide rating is not available or if we can't determine if the house will fit into any of our other categories.