What Are Window Energy Ratings (WER) and How Do They Differ from U-Values?

When comparing new windows, you’ll often see two important terms mentioned — Window Energy Ratings (WER) and U-values. Both measure energy performance, but they look at efficiency in different ways. Understanding the difference helps you choose the right window for comfort, savings, and long-term value.

What Are Window Energy Ratings (WER)?

Window Energy Ratings are a simple way to show how efficient a complete window system is — including the frame, glass, spacer bars, and seals.
They’re graded on a scale from A++ (the most efficient) down to E, following standards set by the British Fenestration Rating Council (BFRC).

This rating reflects how much energy a window keeps in your home compared to how much it lets out.
A higher WER means better insulation, lower heating bills, and reduced carbon footprint.

In short:

• A+ or A rated windows help keep warmth in during winter.

• Lower-rated windows allow more heat loss and can increase your energy use.

What Are U-Values?

U-values measure heat transfer — how much heat passes through the window materials themselves.
They’re expressed in W/m²K (watts per square metre per degree Kelvin).
The lower the U-value, the better the insulation performance.

U-values are based on technical testing and are typically used by architects, builders, and engineers to meet building regulations and energy performance targets.

Key Difference Between WER and U-Values

The main difference is that:

• WER gives you an overall performance rating for the entire window (including solar gain and air leakage).

• U-value focuses purely on the thermal insulation of the materials.

In other words, U-values are a technical measurement, while WER provides a consumer-friendly label that’s easy to compare at a glance.

What to Look for When Buying Windows

When choosing new windows for your home:

• Aim for A-rated or above on the WER scale.

• Check that the U-value is 1.4 W/m²K or lower for compliance with UK building standards.

• Consider triple glazing or thermally broken aluminium frames for even better efficiency.

At Home Build Doors, our range of aluminium and uPVC windows are designed to meet and exceed these standards — offering excellent insulation, sound reduction, and style to match your property.