This standard provides a calculation method for the economical issues of heating systems and other systems that are involved in the energy demand and energy consumption of the building. This standard applies to all types of buildings. The fundamental principles and terminology are explained in this standard. The main items of the standard are: definitions and structure of the types of costs, which shall be taken into account for calculation of the economical efficiency of saving options in buildings; data needed for definition of costs related to systems under consideration; calculation method(s); expression of the result of the economic calculation; informative annexes indicating default values of e. g. lifetime, costs for repair, costs for maintenance, in order to introduce default values for calculations. This standard is part of the method for calculation of economic performance of energy saving options in buildings (e. g. insulation, better performing generators and distribution systems, efficient lighting, renewable sources, combined heat and power). The scope of this specific part is to standardise: required inputs; calculation methods; required outputs for economic calculations of energy systems related to the energy performance of buildings. NOTE Sensitivity of results increases with the number of parameters under consideration (e. g. lifetime, interest rates, development of different types of costs). The more parameters one changes when comparing different solutions, the more difficult it is to draw conclusions from the economic results of the calculations. Economical results are closely related to the specific project under consideration, and no general conclusions should be drawn from any such results.