Because the ER, relies on an average solar gain, it can only be used for general comparisons. It can not be used to compare actual performance for a specific location, orientation and window size. The ERS is calculated the same way as the ER, except it is for a specific location, orientation and window size. Thermotech's Fibertherm windows have the highest ERS of any windows registered with Ontario Hydro. The diagrams below illustrate the ER and ERS for several glass options in the Fibertherm Casement.
Every window maker has slightly different designs.
But no matter what you are considering, no matter what other sales people say, no window registered with Ontario Hydro has a better Rating than Thermotech's Fibertherm Windows. With your questions or comments.