[pomian]

Yes. Makes measurable difference. Especially big picture windows. Other things to consider: is the type of glass. In places like Arizona you may not want sunlight to get through, but further north, the sun can provide a great deal of heat. So you want transmissivity. To stop that in the summer, you have a overhang roof, far enough out, to block sun in summer but not winter. Also, be smart installing the window, to prevent leakage around the frame, lots of good tricks there. For example- Avoid tightness of frame.