The short answer
WERS, the Window Energy Rating Scheme, rates windows 0 to 10 stars↗, with heating and cooling scored separately. Both ratings are built from U-value, SHGC and air infiltration↗. The numbers behind them, 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 | Counted in both star ratings |
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.