The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating properties.

 
The U-factor is the nationally recognized rating method by the National Fenestration Rating Council (NFRC)  for the whole window, including glazing, frame and spacers. Center-of-glass U-factor is also sometimes referenced, and describes the performance of the glazing alone without the effects of the frame. For most energy efficient windows, the whole window U-factor is higher than the center-of-glass U-factor.

High-performance double-pane windows can have U-factors of 0.30 or lower.

Low U-factors are most important in heating dominated climates, although they are also beneficial in cooling dominated climates. ENERGY STAR provides recommended U-factors for your climate; additionally, the Window Selection Tool compares average simulated energy costs for your location based on various window types.

What is the difference between U-factor and R-value?
While the U-factor is used to express the insulation value of windows, R-value is used for insulation in most other parts of the building envelope (walls, floors, roofs). To compare R-value and U-factor, divide 1 by the U-factor number, E.g.: a 0.25 U-factor equals a 1/0.25 = 4 R-value.