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).
To the north, the windows should be shaded in summer by correctly sized eaves. To maximise the benefit of free heat from the winter sun, look for windows with at least four heating stars. This will also ensure that beneficial winter sun is admitted and heat loss is minimised.
Note that occasionally 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 a heating climate should be followed strictly.
In a heating climate, windows with at least four heating stars, on all sides of the house, will ensure that unwanted heat losses or gains are minimised. This means the windows must have a low U-value. 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 composite metal / timber (wood) design, or timber or uPVC frames, outperform standard aluminium windows. This performance difference shows up in the WERS stars.
The final way that heat leaves a 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.