Building scientists look at windows in a much different way than replacement contractors do. To the energy concerned, a window is a hole. It's a weakness in the building envelope, a place where air, heat, and moisture can enter.
If you are serious about selling window replacements on the basis of energy efficiency, it's important you really understand the energy efficiency. This article, from one of the premiere building science voices in the U.S. will help. (And it's not all bad for the window installer.) Read more.