Window Energy Ratings Explained (A++ to E)

That colourful “rainbow” sticker on a window is a Window Energy Rating (WER), run in the UK by the British Fenestration Rating Council (BFRC). It grades a window from A++ (best) down to E, giving you an at-a-glance sense of overall efficiency.

What the rating actually measures

Unlike a U-value, which only measures heat loss, the WER balances three things: heat lost through the window, useful solar heat gained through the glass, and air leakage around the frame. That’s why a window can score well even without the very lowest U-value — it’s capturing free warmth from the sun.

What band should I aim for?

For a replacement window the Building Regulations require at least Band B (or a U-value of 1.4 W/m²K (WER Band B)). Most quality double glazing is rated A or A+ today, so Band B should be your floor, not your target.

Does a higher band save more money?

Moving from old or single glazing to A-rated double glazing delivers the real-world comfort and bill savings. Between A and A++, the differences are smaller and best justified by specific needs — exposure, noise or a new build. When you get quotes, ask each installer to state the WER and U-value so you’re comparing like with like.

Frequently asked questions

Is WER or U-value more important?

They’re complementary. U-value is the cleanest single number for warmth; WER captures the bigger picture including solar gain.

Can doors have energy ratings?

Yes — doors use a Door Energy Rating (DER); replacement doors need Band C or a U-value of 1.4 or better.