What do the Window Energy Ratings (A++ to E) mean?

The Window Energy Rating (WER) is a single traffic-light banded score for the whole window’s energy efficiency, running from A++ (most efficient) down to E. The scheme measures not just the U-value but also solar heat gain (g-value) and air leakage across all the fitted window’s components. Further down the label is the Energy Index, a figure derived by taking the available solar heat gains and subtracting the thermal losses from U-value and air leakage — so a window can earn a higher band either by losing less heat or by gaining more usable solar energy. The headline letter is the quick comparison tool; the supporting figures (thermal transmittance, solar factor, effective air leakage) are printed underneath for anyone who needs the detail. In short: it’s an at-a-glance way to compare whole windows on total energy performance, like the rainbow label on a fridge.