Energy Rating

What Is It?

In the final analysis, the Energy Rating (ER) number is the bottom line for energy-conscious consumers as it is the indicator of the window’s thermal performance. The ER number 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, spacer and glass; and 3) air leakage heat losses. It is a rating solely of a window’s energy performance, regardless of how or with what materials it was built.

The Higher The ER The Better

The ER can be used as an indicator of the effect the windows will have on an annual heating bill. A high ER (over 40) means that the window will add more heat than it loses during the heating season. A lower 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.

Things to Remember

Most fixed windows will have better ER numbers than operable windows. There are two reasons for this. First because the standard test size for a fixed window is larger than most operable windows and thus have more glass area relative to frame area and the frames tend to be narrower because they do not need separate moveable sash. This translates into more solar gains and less frame losses and a higher ER number. Second, fixed windows have less air leakage than operable ones, simply because you can’t open them so the heat loss is less resulting in a higher ER number.

Compare Apples to Apples

The ER system calculates an ER number for a specified window size in each window type. Since the percentage of glass to total window area makes a difference to the ER, window types with smaller frames will get higher ERs. So a fixed picture window will get a higher ER than a slider for example.

Also the test is done at specific sizes. So although it is a good comparison from window A to window B another size will have different energy use.