5.3 Energy terminology

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.


Buildings represent 40% of the energy consumption in the EU. Windows have a substantial impact on the energy consumption in buildings and on the indoor environment. However, the effect can be both positive and negative and care must be taken to use the advantages of windows and avoid the disadvantages.

For more information, see section 2.7.1. Energy balance refers to a single window and is expressed in kWh/m² per year for the window. The value expresses the energy efficiency of the window alone and can be used to compare different windows with regard to type, size, pane and other parameters. For more information on this subject, see section 5.5.3.
VELUX Group (2009) VELUX Energy Terminology Guide