Are you thinking about improving thermal efficiency of your home?
Then, as the first step, consider better insulation around your house, be it roof/wall/ceiling insulation, single glazed windows, fans and vents, or other causes.
In winter, an average Australian home can lose up to 40% of its heating energy through windows and gain twice more in summer or up to 87% (Source: yourhome.gov.au).
To put this in perspective, it is useful to compare U-values of common window types found in Melbourne today. With windows, the lower the U-value the better window insulates against heat.
Typical single glazed aluminium window with 3mm glass has a U-value of 7.4
Typical single glazed timber window with 3mm glass has a U-value of 5.5
Windows Republic uPVC double glazed window with Low-E glass
has a U-value of 1.9