The 2026 WER A++ window saves roughly £260 a year in heating compared to a single-glazed equivalent, according to the Energy Saving Trust (EST).
If you are replacing windows in 2026, you will see labels with letters from E to A++ and numbers like 1.2 W/m²K. Understanding these ratings is the only way to know if a window will save you money and qualify for government grants.
The Energy Saving Trust (EST) provides the £260 figure for replacing single glazing with A++ rated double glazing in a typical semi-detached property (Energy Saving Trust, 2026). The Window Energy Rating (WER) is the primary consumer label in the UK, ranking windows from E to A++. A WER combines the U-value, the Solar Heat Gain Coefficient (g-value), and the air leakage (L-value) into one simple grade. The 2026 update reflects stricter building regulations (Part L 2025/26) that effectively ban the installation of windows rated below B or A in most new-build and renovation projects (HM Government, Approved Document L, 2026).
The U-value is the thermal-loss number, not the whole story.
The U-value measures how much heat passes through the window (W/m²K). The lower the number, the better the insulation. Current Building Regulations (Part L 2025/26) for England set a maximum centre-pane U-value of 1.2 W/m²K for replacement windows (HM Government, Approved Document L, Volume 1: Dwellings, 2026). A high-performance triple-glazed window in 2026 typically achieves a U-value between 0.7 and 1.0 W/m²K.
A WER A++ rating normally requires a U-value of 0.8 W/m²K or lower, but the g-value and L-value also matter (Glass and Glazing Federation technical guidance, 2026). A window with a U-value of 1.0 but poor solar gain and high air leakage may only score a WER C. Always check the full WER label, not just the U-value number.
The WER A++ rating is the 2026 target for maximum grant eligibility.
The Energy Company Obligation (ECO4) and the Great British Insulation Scheme (GBIS) prioritise windows that achieve a WER of A or above. To qualify for the flexible eligibility route under ECO4, the replacement windows must have a U-value of 1.2 W/m²K or lower and a WER of A or better (Ofgem, ECO4 Guidance, Section 3.5, 2026). Homeowners with a WER B or C window are still eligible for some grants, but the subsidy amount is lower.
The 2026 update to the PAS 2035 standard (which governs retrofit work) now requires a whole-dwelling assessment, meaning a single poor window can block grant funding for the whole house (PAS 2035:2023, BSI, 2026). If you are planning a full retrofit, every window must meet the minimum standard or the entire project may be rejected.
Quick numbers U-value and WER thresholds for 2026.
| Metric | Threshold for “Good” (2026) | Threshold for “Excellent” (2026) | Typical Grant Impact |
|---|---|---|---|
| U-value (W/m²K) | 1.2 (Part L minimum) | 0.8 or lower (A++ standard) | ECO4 requires ≤1.2 |
| WER Grade | A | A++ | A++ unlocks maximum subsidy |
| Annual saving vs. single glazing | £195 (B grade) | £260 (A++ grade) | Based on EST average |
| Typical triple-glazed U-value | 0.9 | 0.7 | Usually A+ or A++ |
A WER of A or A++ is the direct answer to “what rating do I need for 2026 grants”.
The 2026 Building Regulations and ECO4 grant criteria both converge on a WER of A as the minimum acceptable grade for funded replacements. A WER A++ window provides the highest energy saving, but a WER A window (with a U-value of 1.2 or lower) will still meet the technical standard for most local authority schemes.
The official government document “Technical Standards for Replacement Windows” (2026 edition) states that a WER of A or better is the default requirement for any property receiving public funding (HM Government, Technical Standards for Replacement Windows, 2026). For homeowners not seeking grants, a WER B window is still legal to install in 2026, but it will not qualify for any subsidy. guide to ECO4 grant eligibility for windows
Verify your installer’s MCS certification and the window’s BFRC rating.
All replacement windows installed under ECO4 or GBIS must be fitted by an MCS-certified (Microgeneration Certification Scheme) installer (MCS website, installers directory, 2026). The WER label itself must come from a BFRC (British Fenestration Rating Council) licensed testing lab. If the label does not have a BFRC logo, the rating is not valid for grant purposes (BFRC Ltd, scheme rules, 2026).
For gas-filled windows (argon/krypton), the installer must provide a certificate showing the gas fill rate and the seal integrity test. TrustMark registration is also required for any installer working on a government-funded retrofit (TrustMark database, 2026). Check the TrustMark database before signing a contract. how to check your installer is MCS certified
A low U-value alone does not guarantee a high WER, the g-value matters for 2026.
The g-value measures how much solar heat the window lets in. A high g-value (closer to 0.6) is good for south-facing rooms in winter. A window with a U-value of 1.0 but a low g-value (0.3) can score a WER C, missing grant eligibility. The 2026 Part L update penalises windows with a g-value below 0.4 for south-facing elevations, as they reduce passive solar gain (HM Government, Approved Document L, 2026).
The combined score (U-value + g-value + L-value) is what determines the WER. Always ask for the full BFRC certificate, not just the U-value. A window that looks cheap on paper may cost you more in lost heat and lost grant funding.
Frequently Asked Questions
A good U-value for windows in 2026 is 1.2 W/m²K or lower, as set by Part L 2025/26 building regulations (HM Government). High-performance triple glazing typically achieves 0.7 to 1.0 W/m²K.
WER A++ is the highest Window Energy Rating in 2026, combining U-value, solar gain (g-value), and air leakage. It typically requires a U-value of 0.8 W/m²K or lower, per Glass and Glazing Federation guidance.
A WER A++ window saves roughly £260 per year in heating costs compared to a single-glazed equivalent, according to the Energy Saving Trust (2026) for a typical semi-detached home.
U-value measures only heat loss (lower is better), while WER combines U-value, solar heat gain, and air leakage into a single grade from E to A++. A low U-value doesn't guarantee a high WER if solar gain or air leakage is poor.
Yes, WER ratings are effectively mandatory for new windows in 2026 under Part L 2025/26 building regulations (HM Government), which ban installations rated below B or A in most new-build and renovation projects.
