Nearly zero energy buildings are to become a requirement as part of the European energy policy. There are many ways of
designing nearly zero energy buildings, but there is a lack of knowledge on how to end up with the most economical optimal
solution. Therefore this paper present a method for finding the economical optimal solutions based on the use of the cost of
conserved energy for each main building envelope part and building service system and cost of produced energy for each
energy producing system. By use of information on construction cost and developed models of the yearly energy use for each
component, a function is set up that represents the relation of the marginal cost of conserved energy and the energy use for
different quantities and qualities of the components.
The optimal mix of solutions for the whole building is found by selecting building parts with the same cost of conserved energy.
The constraint is that the total energy performance of the building is fulfilling the requirements. A case example shows how the
method with success can find the solution for a typical single family house with the energy performance requirement for 2020.