In other words, a net zero energy building is a building with a very high energy performance whose very high level of energy efficiency means that the overall annual primary energy consumption is equal to or less than the energy production from the renewable energy source on-site. This does not mean that the Treurenberg does not need energy but that its total annual primary energy consumption is equal to zero.
The net energy demand of the Treurenberg is 5 kWh/m2.year, which corresponds to the energy of a 25W bulb used for one hour per day for each square meter. This is three times less than a passive building!
The primary energy consumption of the Treurenberg is 36kwh/m2.year, which corresponds to the amount of energy consumed by a 22.5 W (4) bulb used every day throughout the day for each square meter of the building. This is two times less than a passive building and twelve times less than an everage office building in Brussels. To compensate for this energy consumption, solar panels are placed on its three facades and on the roof and their annual production will completely offset its annual consumption.
The building consumes no power over an entire year!!! (Excluding consumption of private facilities (office equipment, kitchenette, etc. ), lifts, surfaces outside the protected volume (e.g. car parks) and retail outlets.
More info at : http://www.treurenberg.be/net-zero-energy