Radiant heat gain occurs when solar energy is transmitted through the glass. In addition, some solar energy enters indirectly. This occurs when the glass (and frame) are exposed to sunlight and heat up. Some of this heat is transferred inside by conduction and convection (air currents).
The best way to stop radiant solar heat heat gain is to shade the glass, but this is not always possible. In addition, shading systems sometimes interfere with the view, reduce natural light, inhibit ventilation or add cost to the window system. Every situation is unique. An alternative is to select a window with a built-in, low solar heat gain coefficient. Under WERS, windows with more than three cooling stars have a low solar heat gain coefficient. A poor cooling star rating tells us that the window, on its own, 'needs help' to reduce solar heat gain. A good cooling rating implies the window reduces solar heat gain very substantially and may need no further shading treatment.
Note that even in a heating climate, it may be desirable to specify a good 'cooling' window if the orientation is to the west and the window is large. However for other elevations, the guidelines for that climate type should be followed.
To reduce unwanted conducted heat gain in a cooling climate, a window with a low U-value is desirable, especially if the house is air-conditioned. The best 'cooling' windows have low U-values as well as low solar heat gain coefficients. If in doubt, look for a low U-value on the WERS certificate. Insulating glass units (IGUs) with some sort of low-e coating and an argon gas fill between the panes give the lowest U-values: as low as 1.8, compared with at least 5 in the case of a single-glazed clear window. A frame with a low U-value assists in reducing the whole-window U-value. Frames that use thermal breaks or other improvements, or timber or uPVC frames, outperform standard aluminium windows. This performance difference shows up in the WERS stars.
The final way that unwanted heat enters the home is through gaps and cracks around sashes and frames. When a window is shut it should be shut. WERS-rated windows must satisfy Australian Standard AS 2047 for air infiltration performance. Most easily exceed it and achieve air leakage figures below 1 litre per second per square metre of window area. Look for windows with good weather seals. Traditionally, those with 'compression' seals, as fitted to awning and casement windows, tended to have superior long-term infitration performance. However recent advances in some sliding window seals have reduced the gap.