Thank Congress for passing the Energy Policy Act of 2005 (EPACT) which, among many other things, offered homeowners a 10% tax credit for energy saving renovations to their homes. I looked at EPACT when it was first passed  because I knew somewhere down the road I’d need to replace my windows and sliding doors.
If you are replacing your windows, you can receive a 10% tax credit up to $200 for the job if the windows meet 2000 IECC & Amendments specifications. If you are replacing exterior doors, which sliding doors count as, then your cap is raised to $500. The quote of $6500 for the whole job means I’ll be able to claim a tax credit of $500, reducing the job to a mere $6000. (it’s still scary)
For that tax credit and an explanation of the others, visit the official Dept. of Energy website on EPACT .