Performance · 5 min read · Updated 2026
U-values and Window Energy Ratings explained
Two numbers get quoted constantly, and they are not measuring the same thing. Knowing which is which stops you being sold a figure that flatters the product.
U-value
A U-value measures heat loss in watts per square metre per degree of temperature difference. Lower is better.
Crucially, ask for the whole-window U-value, not the centre-pane figure. Centre-pane ignores the frame and the edge of the unit, where most of the loss actually happens, and always looks better.
Window Energy Rating (WER)
The A++ to E scale used in the UK combines heat loss, solar gain and air leakage into a single letter. It is a useful shorthand but hides detail — two A-rated windows can behave quite differently.
What to specify
For a replacement window, Building Regulations in England require a whole-window U-value no worse than 1.4 W/m²K, or WER band B. Most decent systems beat that comfortably, so treat it as a floor rather than a target.