The FTC has warned several replacement window manufacturers about their stated claims for energy savings. Anyone who has paid attention to some of the claims would likely question them if they thought about it just a bit.
Many claims state that by replacing windows, homeowners might see a savings of anywhere from 20-40% on their energy bills. My sense all along has been that those kinds of statements might be a little ambitious. Apparently the FTC has also decided that and has been taking steps to reign in some of the marketing by various window manufactures when it comes to replacement windows.