The lower the U-Value the better the door/window will insulate. Doors/windows with a U-Value of 3.1 will be twice as energy efficient as a window with a U-Value of 6.2.This means you should use approximately half the power to heat or cool your house.
Explaining Solar Heat Gain Co-efficient (SHGC)
Mainly used where excessive sunlight will enter house.The lower the SHGC the less solar heat that will enter through the door or window. West facing windows need low SHGC as they generally need to block out more heat. North windows should have high SHGC.