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.

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Grassmoor Glass  /  Unit 1-3 /  Circular Road  /  Storforth Lane Industrial Estate  /  Hasland  /  Chesterfield  /  Derbyshire  /   S41 OQD

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Open 9am - 5pm Monday to Friday

9am to 12pm on Saturday


Telephone: 01246 233100

sales@grassmoorglass.co.uk