It doesn't.

Ask this question in the windows forum and you will should get some interesting answers from some very professional and knowledgeable window folks.

sorry. i thought it would be an energy question. i see forums for heating, windows, remodeling, and so on. figured most home energy folks would know a lot about ways to improve house energy use thru improvements like better windows.

When I had a local window company come out for an estimate I was ready with last winter heating gas bills($800 for November thru April)

Salesperson said new windows will save 40%. I pulled out gas bills from folder and showed I spent $800 last winter season. Providing we keep the thermostat at 68 and have a similar winter season I asked if he could guaranty this seasons bills would be no more than $480.

His short answer was they can't make that kind of guaranty but can only suggest the savings.

you can not, its impossible. The difference bewteen a standard low E double pane and say a triple pane filled window is just a couple R, and the u is anywhere from .05-.08 lower. It really comes down then to air infiltration and the install. %5-10 savings is even a big stretch. Also keep in mind they compare those savings to houses that keep the t stats at 90 and have all the windows open ;)