The rating formula used by the Thermal Rating Register was determined by following the methodology adopted by the European Window Energy Rating Scheme, EWERS, which was developed as part of the EU 'Save' initiative. This is an agreed standard for UK Building Regulation compliance.
The coefficients (weather conditions, solar gain/loss ) used in the rating formula are based on the energy balance of a notional house through an average UK year, with notional window sizes and configurations.
The performance of a real window, in a real building, will differ from that of the notional window used for the rating calculation. In some cases the performance will be better and in others it will be worse. The objective of the Thermal Rating is to allow a relative comparison of the overall thermal efficiency of different window systems, comparing how the windows perform under standard conditions.
The use of a window system with a good Thermal Rating does not guarantee that a home will be easy to heat during winter, and may lead to issues with solar overheating in summer unless a proper ventilation strategy is adopted.
Things to consider:
Now consider your home and how you heat it!