What is a window energy rating (WER)?

A window energy rating is a measure of the overall energy performance of a window, combining heat loss (U-value), solar heat gain (g-value) and air leakage into a single rating. The BFRC (British Fenestration Rating Council) administers the scheme in the UK, rating windows from A++ (most efficient) to E (least efficient). It shows the net energy balance in kWh/m²/year.

What WER band do I need for Building Regulations?

For replacement windows under Part L of UK Building Regulations, you need either a whole-window U-value of 1.4 W/m²K or better, OR a window energy rating of band B or above. For new build, the standard is higher at 1.2 W/m²K U-value. The WER route can sometimes be easier to achieve because solar gain offsets heat loss.

What is the g-value of a window?

The g-value (solar factor or total solar energy transmittance) measures how much solar energy passes through the glass. A g-value of 0.5 means 50% of solar energy enters the building. Higher g-values mean more free solar heating in winter but potentially more overheating in summer. Typical values: clear double glazing ~0.63, low-E double ~0.43, triple glazing ~0.35.

How is the WER calculated?

The BFRC formula is: WER = (218.6 x g) - (68.5 x Uw) - (L x 0.5), measured in kWh/m²/year. The 218.6 constant represents the solar irradiance benefit averaged across UK orientations over a heating season. The 68.5 represents the degree-hour heat loss. The result shows the net annual energy contribution of the window per square metre.

Can a window have a positive energy rating?

Yes. A positive WER means the window gains more energy from solar radiation than it loses through conduction and air leakage over a heating season. This is achievable with good U-values (below 1.2), reasonable g-values (above 0.40) and low air leakage. A++ rated windows typically achieve +20 kWh/m²/year or better.

TradeCalculator.co.uk

Window Energy Rating Calculator — WER Band Estimator

Estimate the BFRC window energy rating band (A++ to E) from U-value, solar gain and air leakage. Check Part L compliance for replacement windows.

Window U-Value (Uw)

Whole-window U-value

Solar Gain (g-value)

Total solar energy transmittance (0-1)

Air Leakage

At 600Pa test pressure

Window Type

For reference only

How We Calculate This

This calculator uses the BFRC (British Fenestration Rating Council) methodology to estimate the window energy rating from three key parameters.

The BFRC formula

WER = (218.6 × g) - (68.5 × Uw) - (L × 0.5)

g = solar energy transmittance (g-value, 0 to 1)
Uw = whole-window U-value (W/m²K)
L = air leakage at 600Pa (m³/hr/m²)

WER bands

A++ ≥ +20 kWh/m²/yr
A+ ≥ +10 kWh/m²/yr
A ≥ 0 kWh/m²/yr
B ≥ -10 kWh/m²/yr
C ≥ -20, D ≥ -30, E < -30

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.