This Standard is to standardise the required inputs, the outputs and the links (structure) of the calculation method in order to achieve a common European calculation method. A common calculation method will help to facilitate the free circulation of services (optimisation of the energy performance, display of conventional level of energy requirements) and products (software). Ventilation systems are not included in this standard (e.g. balanced systems with heat recovery), but if the air is preheated or an air heating system is installed, system losses of these systems are covered by this standard. The energy performance may be assessed either by values of the system efficiencies or by values of the system losses due to inefficiencies.