Ratings Explained
A number of factors influence the energy efficiency of the window. These include the profile system and the specification of a sealed unit.
Windows are rated from A+ (no heat loss at all) through to G (extremely energy inefficient), the window Energy Rating label allows you to see at a glance how energy efficient a window is.
Windows Energy Ratings are calculated using various values:
- Thermal Transmittance (U value)
- Solar factor (G value)
- Air Leakage (L value)
The thermal transmittance or U-value represents how much heat can escape through the glass. This is measured in Kilowatt-hours, per square meter, per year.
For example: 3.5kWh/m2.K - This means that 3.5 kilowatts of energy is lost through every square metre of the window every year
The solar factor or G-value represents how much energy is gained through your windows from the Sun. This is measured in kilowatt hours per metre, per year.
The Air Leakage or L-value represents exactly what you would think, how much air is leaked through the windows (the lower the better). This is measured in kilowatt hours per metre, per year.