I see the same types of questions come through this great site over and over again.
Go to the NFRC site and read about windows.
THEN, insist that your sales guy provide you with the unique NFRC# associated with the window they are trying to get you to dump thousands of dollars on.
Look up that product and then compare with other products you've done your homework on and choose a window like an informed consumer-it is all we have to go on.
If they don't know what you are talking about or say the number doesn't exist or not important [or the famliar line "they choose one window from the assembly line and use that as a representative sample so the ratings are not representative"], for their product then it isn't energy star compliant and not worth buying and that painful presentaion was a waste of time......UP FRONT: BEFORE THESE GUYS BEND YOUR EAR ABOUT THEIR PRODUCT-INSIST THAT THEY SHOW UP WITH NFRC PRODUCT NUMBERS BEFORE YOU GIVE THEM AN AUDIENCE....if not walk away and call someone else.
I knew nothing about windows when buying them as do most of the window buying consumers until this board pointed me to a number of resources.....i have forgotten a lot of what I learned....but do your self a flavor and do your homework.....I bought some fantastic windows for our home and they passed the test almost immediately....