Zero- and plus- energy houses are buildings that typically require very little energy (for heating, ventilation, home appliances), equipped with their own systems of energy generation (heat and electricity).
Those buildings that produce a surplus of energy are referred to as plus-energy houses.
As opposed to passive buildings, there is no standardized definition of zero-energy buildings. In fact, an ordinary, traditional house equipped with large solar panels and photovoltaic panels could be a zero-energy building, but that makes no ecological or economic sense. Designing buildings in LEB or VLEB standards (passive buildings) with energy-generating systems using renewable energy sources seems to be the most sensible solution.
Zero-energy buildings fall into two categories: independent of energy networks (off-grid) or those that use external networks as a bufor (on-grid). Typically systems generating energy produce it at a time when there is the least need for it – in the summer and during the day. In contrast, the greatest demand for energy in the home is in the mornings, evenings and winter months (shorter days). Therefore, surplus energy generated can be
transferred to an external network or stored in tanks. Electricity storage is difficult and economically disadvantageous (storage costs as much as buying more energy), external power grid provides the best solution of these problems.