According to the U.S. Department of Energy, leaky and inefficient windows, skylights and doors account for up to 25 percent of the average household's energy bills. Some sources estimate as high as 40 percent. A lot depends on where you live:
The colder or hotter the climate, the greater your heating or cooling costs and the greater potential you have to save money on energy costs. Nearly everyone can benefit by replacing leaky, inefficient windows with modern energy-efficient windows. Depending on your location, you can cut energy costs by as much as 15 percent.
Link to information from Pacific Gas & Electric-
Especially an older home, like our customer's in the picture below, benefits are even greater in terms of energy savings versus cost.