What is a Window Energy Rating? – By double glazing Edinburgh experts
The WER rainbow label you see on certified windows uses the same colour coding as the energy labels on household appliances, running from green at the A end through to red at the bottom of the scale. In this context, it is given to windows on their energy rating.
Each rated window is assigned a band on a scale from A to G, where A is the best-performing band and G the lowest.
How is a WER rating calculated?
The WER is an energy balance calculation rather than a simple heat loss figure, and that distinction matters when you’re comparing specifications.
A glazed window doesn’t only lose heat.
During daylight hours it also captures solar heat from sunlight, and that solar thermal energy contributes positively to the warmth of a room.
The WER takes three factors together:
Solar heat gain
Solar gain is the useful heat that passes through the glass from sunlight. The glass specification and the coating on the inner pane both affect how much solar thermal energy comes through.
The g-value measures how much of the sun’s energy passes through the glass, on a scale from 0 to 1. A higher g-value means more warmth enters the room from sunlight, contributing positively to the window’s WER band.
A window that captures more solar heat requires less from your boiler to maintain room temperature, which is why solar gain features in the WER calculation as a positive contribution.
Heat loss (U-value)
The U-value of the whole window measures how much heat passes through the glass, window frame, and edge seal in watts per square metre per kelvin. The lower the U-value, the less heat escapes when it’s cold outside.
A-rated double glazing achieves around 1.2 W/m²K.
Our triple glazed windows using the Advanced 70+ uPVC profile reach 0.8 W/m²K.
To lower the U-value further requires improvements to both the glass specification and the frame design, and our profile delivers both.
Air leakage
Air leakage measures cold air infiltrating around the frame edges and warm air escaping. New windows with compression seals at every edge perform significantly better than older frames where the seals have degraded or the frame has warped over time.
The WER combines all three factors into a single energy balance figure.
A window with strong solar heat gain can offset a slightly higher U-value, and a well-sealed frame contributes meaningfully to the overall energy efficiency score. The balance of the three elements together determines the band.
The WER bands: what A to G means for energy efficiency
Understanding window energy ratings becomes straightforward once you know what the scale represents. The rating scale runs from A (the highest-performing band) down to G.
Windows with an energy rating of band C are the minimum required by UK building regulations for replacement glazing. C rating is compliant, but it sits well below what A-rated glazing delivers.
The actual energy consumption difference between C-rated windows and A-rated windows is measurable, and across a Scottish heating season that gap adds up on your bills.
A window with a high WER score delivers lower heat loss, better solar heat capture, and less air infiltration than a lower-rated unit. The higher the rating, the closer the window gets to a net positive energy balance across the year.
C rated windows are not a poor product in isolation, but the gap between a C rating and an A rating is substantial.
Our energy efficient windows carry A-rated WER across the full range.
We don’t supply anything lower, because the rating is part of the specification we’ve built the range around.
WER vs U-values: what’s the difference?
Both figures appear regularly in window specifications, and they measure different things.
The U-value measures thermal transmittance. This is how much heat passes through the whole window, including heat lost through thermal bridging at the spacer bar and frame edges. It only covers what leaves the building through the glass, frame, and edge.
The WER uses the U-value as one component of a larger energy balance calculation. It also includes solar heat gain and air leakage, so the WER reflects the overall energy efficiency of the window rather than just its insulation value.
For choosing between different window products, the WER gives you the complete picture. The U-value is useful for building regulations compliance and energy performance certificates, but the WER tells you how the window will actually perform across an average year in the UK.
A window with a good U-value but low solar transmittance might score lower on the WER than a window with a slightly higher U-value that captures more useful solar gain. That’s why looking at both figures together gives a better window specification than either figure alone.
Energy efficient windows and doors across Scotland
Every window we supply carries an A-rated WER as minimum. Low-E glass and argon gas fill are standard on every unit, and our Advanced 70+ uPVC profile achieves U-values as low as 0.8 W/m²K.
Our performing windows meet the top band of the rating scale as a matter of course, not as an optional upgrade.
We’ve been supplying and fitting uPVC windows throughout Scotland since 1988. Every installation comes with a 15-year guarantee covering the sealed glass unit, frames, hardware, and workmanship.
Call our Edinburgh number today on 0131 370 0444 or request a free quote online. We’ll arrange a no-obligation home survey and talk through the double and triple glazing options suited to your property.
