The short answer
WERS, the Window Energy Rating Scheme, rates windows on a scale of 0 to 10 stars, with heating and cooling scored separately. Behind the stars, the scheme lists the window's U value, SHGC and air infiltration. The two numbers that matter most, Uw and SHGCw, describe the whole window rather than the glass alone, which is what makes them comparable between products.
Why are there two star ratings?
Because a window that is good at holding heat in during a Hobart winter is not automatically good at keeping heat out during a Perth summer. WERS rates the heating case and the cooling case separately, and ranks each against a scale built from 17 generic window types that run from very low to very high performance. A product can look strong on one and ordinary on the other, so read both before you decide.
What is the difference between Uw and SHGCw?
| Measure | What it tells you | What to look for |
|---|---|---|
| Uw | Heat conducted through the whole window, frame included | Lower is better in every climate |
| SHGCw | The share of the sun's heat the window lets in | Depends on climate and which way the window faces |
| Air infiltration | Air leaking past the seals | Listed by WERS alongside the stars; lower means fewer draughts |
There is no single right SHGC. The building code itself sets solar heat gain limits by climate zone, across climate zones 1 to 7, which is the clearest sign that the answer changes with where you live and which way the glass faces.
How much does the glazing itself move the U-value?
Reported whole-window U-values
Show the numbers
| Item | Range |
|---|---|
| Single glazed aluminium | 5.8 W/m²K, lower is better |
| Double glazed uPVC | 1.6 to 1.7 W/m²K, lower is better |
| Triple glazed uPVC | 1.2 to 1.4 W/m²K, lower is better |
Those are a manufacturer's figures for its own products, not WERS ratings, and we have labelled them that way in the sources. They are useful for scale. For a purchase, use the rating that belongs to the exact window.
Who does the rating?
Not the window maker. To take part, makers must get ratings from AFRC-accredited raters, which is the point of the scheme: the number on the label comes from an accredited rater rather than a brochure. If a salesperson quotes a performance figure with no rating behind it, ask which product it belongs to and who rated it.
What to ask a supplier for
- The WERS heating and cooling stars for the exact product and configuration being quoted.
- The whole-window Uw and SHGCw, not a centre-of-glass number.
- Confirmation that the rating covers the frame you are buying, since frame and glass are rated together.
- The same details from every supplier, for the same window size, so the comparison means something.
If you sell or install windows, homeowners will look for these numbers on your own site before they ring. Our website plan for glaziers is built to put them there, product by product.