In the final analysis, the Energy Rating (ER) number is the bottom line for energy-conscious consumers as it is the indicator of the windowΓÇÖs thermal performance during the heating season. The ER is not a temperature rating. It is a scale rating the comparative performance of windows based on three factors: 1) solar heat gains; 2) heat loss through frames, spacers and glass; and 3) air leakage heat losses, regardless of how or with what materials it was built.
The Higher The Number The Better
The ER can be used as an indicator of the effect the windows will have on an annual heating bill. A positive ER means that the window will add more heat than it loses during the heating season. A negative ER means that the window loses more heat than they gain during the heat season. The lower the number, the more heat is lost and the higher the heating costs. The ER of an R20 wall is about -6.
