Energy labeling of windows has been introduced in some Member States of European Union - for example Denmark, Finland, Slovakia and the UK. The UK BFRC scheme on window energy labeling has proved very efficient in communicating to the general public about the energy saving properties of high performance products thus contributing to their faster uptake. Slovak labeling system was introduced in 2008. The paper deals with main principles of energy rating system used in SLOVENERGOokno. National system for rating the energy efficiency of windows and is recognized within the Building Regulations as a method to show compliance for your replacement windows installation.