The energy label on windows works like appliance ratings — but the calculation behind it is worth understanding.
Window Energy Ratings (WER) run from A++ (best) through A+, A, B, C, D and E to G (worst). The scheme is administered by the British Fenestration Rating Council (BFRC), and comparable schemes are run by Certass and BSI. All calculate similarly and the letters are directly comparable.
The rating combines three variables: U-value (heat loss out), G-value / solar factor (heat gain in from sunlight), and L-value (air leakage). Higher solar gain earns credits, higher heat loss loses them. A well-balanced window with low U-value and reasonable solar gain scores A.
A ratings satisfy Building Regs comfortably. A+ typically requires either a very low U-value (triple glazing) or improved solar gain glass. A++ requires both. In practical UK-climate terms, an A-rated double-glazed window is excellent — the marginal energy benefit of A+ over A is measured in single-digit pounds per year.
Ask for the WER on each window in the quote (they can differ between north- and south-facing units if you specify solar-control glass on southern elevations). Compare like-for-like ratings across quotes rather than headline product names. A B-rated window from a premium brand is worse thermally than an A-rated window from a good mid-market brand.
The WER doesn't consider the installation quality — a poorly fitted A-rated window will underperform a well-fitted C-rated one. Rating is necessary but not sufficient. Choose an A-rated window and an installer who takes the survey and installation seriously.
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