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Window Energy Ratings

All our PVC-U windows are rated for their energy efficiency. Many people will be familiar with the ratings that are given to white goods such as fridges and washing machines, which are graded between A and G, depending on their environmental efficiency – A being the best and most efficient. What is less well known is that the same system can be applied to windows as well.

Each window unit comes with a label that sets out its environmental performance in five key areas:

Rating Value (A-G)

This is the most recognisable and visible element of the label, showing clearly the window’s energy efficiency in coloured bands, from the dark green A (the best rating) through to the dark red G (the lowest rating and therefore the least energy efficient). Obviously, windows valued at A are the most preferable.

Energy Index (kWh/m2/year)

This is one of the most important values, as it sets out the amount of energy lost or gained by a window. The value is displayed in terms of kilowatt-hours per square metre of glazing per year. Energy lost is displayed as a negative figure, and energy gained by a positive number.

For example, 1kWh/m2/year means that the window gains a kilowatt of energy for each square metre of glazing over the course of a year; whereas a value of -1kWh/m2/year means that the window loses a kilowatt of energy per square metre in a year.

So the higher the number, the greater the energy conserved – and the more money saved.

Thermal Transmittance (U-value)

This indicates the amount of heat energy lost through a window, set out in kilowatt-hours per square metre of glazing per year, and is a contributing factor to the Energy Index value above. The lower the U-value, the less energy is lost, and the more efficient and economical the unit.

Solar Factor (G-value)

This represents the amount of solar energy that is gained through the window over the course of a year, once again in terms of kilowatt-hours per square metre. The higher the value, the more energy gained and the greater the saving.

Effective Air Leakage (L-value)

This is the amount of air that is leaked through the window over the course of a year, also a contributing factor to the Energy Index rating above. The modern sealed unit should have a zero-rated value, meaning no air leakage and no energy lost.