The VELUX Group’s current terminology on energy use and windows includes two concepts: Energy Performance and Energy Balance (VELUX Group 2009).
Energy performance refers to the total yearly energy demand of a building, including heating, cooling, hot water and electric lighting (household appliances or other electrical equipment are not
included). Energy performance is often expressed in kWh per year per m² of heated floor area (kWh/m²). The lower the value, the better. Energy performance can be used to find the difference between two scenarios, e.g. impact of more or fewer VELUX roof windows on the energy performance
of a building. It can be calculated with dynamic simulation tools, among others the VELUX Energy and Indoor Climate Visualizer.