A hundred years ago, awnings were common on hotels, office buildings, and the grandest of houses, for a very good reason: they shaded the window and kept the heat of the sun out. They fell out of favour when the preferred method of dealing with too much solar energy became the application of electrical energy through air conditioning. They were also a high-maintenance item; it probably took quite a few workers to put them up every spring on some of the bigger installations.
According to the Washington Post, The Department of Energy estimates that awnings can reduce solar heat gain—the amount temperature rises because of sunshine—by as much as 65 percent on windows with southern exposures and 77 percent on those with western exposures. Your furniture will last longer, too.
According to the Professional Awning Manufacturers Association , a totally unbiased source, window awnings can reduce household cooling energy by as much as 26 percent in hot climates, and 33 percent in cold climates.