Similarly, a window on the north side of a house in a cold climate should be designed to save energy. Those on the south side should save energy and allow as much solar energy as possible to pass through. Obviously, the entire window R value is a better indicator of performance. Don't be confused if you see a U value alongside the familiar R rating. They are different sides of the same coin.
R values measure resistance to heat transfer - the higher the better.
U values measure heat transfer - the lower the better. To keep everything straight, convert U values to R values by dividing the U value into 1. This rating tells how much solar energy a window captures. A single pane of glass has a shading coefficient of 1. A blocked window would be rated at 0.