A Net Zero Energy (NZE) building needs to achieve a balance between the on-site production of energy and the energy it consumes over the course of one year. A more efficient home will require less energy to be produced, while a home with high energy requirements will need a larger production capacity.
A Net Zero Energy building is not necessarily an off-the-grid home. In order to respond to peak demands and periods of lower energy production (such as reduced sunlight in winter, for solar), a NZE building may purchase energy from, and sell back to the grid as required.