The energy performance of a window depends on how well these materials stop heat from passing through, as well as how much sunlight travels through the glass and how little air can leak around the window.
If you are thinking about changing your windows or doors you will need to think about U-values and the energy rating.
A U-value measures the rate at which heat escapes through a fabric. The lower the figure, the better. A U-value of zero means that no heat is escaping.
The energy-rating scale goes from A++ to E (with A++ being the best). The entire window (the frame and the glass) is assessed for heat loss, draughts and solar gain, giving a rating that indicates the overall impact of fitting that window in your home.