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

When choosing new windows or doors, you’ll often see terms like energy rating and U-value.
Both measure how well a product keeps heat inside your home — but they do it in different ways. Understanding the difference helps you choose designs that save energy, lower heating costs, and keep your home comfortable all year round.

What Is a Window Energy Rating (WER)?

The Window Energy Rating (WER) is an overall score that shows how energy-efficient a window is.
It’s based on the familiar A++ to G scale, where A++ is the best and G is the least efficient.

The rating is calculated using three factors:

• U-value – how well the window keeps heat in

• Solar gain – how much natural warmth from sunlight it lets in

• Air leakage – how airtight the seals and frame are

WER gives you an easy-to-understand comparison between different windows, so you can quickly identify which products offer the best performance.

What Is a U-Value?

A U-value measures the rate of heat transfer through a material — essentially, how much warmth escapes through your window.
It’s expressed as W/m²K (watts per square metre per degree Kelvin).

The lower the U-value, the better the insulation.

• 1.2 W/m²K or lower is excellent for double glazing

• 0.8 W/m²K or lower is exceptional and typical of triple glazing

U-values are often used by architects and builders because they provide precise technical detail about thermal performance.

WER vs U-Value: The Key Differences

In simple terms:

• WER gives an overall view of energy efficiency.

• U-value provides a detailed measurement of insulation.

For most homeowners, WER is the easier way to compare products — but both are important for long-term energy savings.

Why These Ratings Matter

Windows and doors with strong energy performance help to:

• Keep your home warmer in winter and cooler in summer

• Lower your energy bills

• Reduce your carbon footprint

• Improve comfort and sound insulation

At Home Build Doors, our aluminium, composite, and flush PVC ranges are engineered to deliver high WER ratings and low U-values — combining sleek design with exceptional efficiency.

Ready to Upgrade?

Explore our full range of high-performance windows and doors at Home Build Doors and discover how better insulation can enhance comfort, reduce energy use, and transform your home.

Start your quote today at homebuilddoors.co.uk.