The energy star tax credit is only available to home owners who install windows that have a U-Value of .30 or less. Gorell makes a good window but like most manufactures make them in various grades and qualitys. The best bet for any home owner who wants to make sure that they are getting a good performance product is to check the U-value and make sure your contractor specifies the the u-value on your contract. .35 u-value gets the window an energy star lable, but no tax credit unless the nfrc ratings are .30 or less.
Source: http://www.tri-pane.com