How do I determine if the energy savings are worth replacing windows for?
I have been told but have no first hand knowledge that the way low E windows work is the coating is somewhat directional in the sense that they reflect summer solar energy because the sun in higher in the sky.Winter solar energy is allowed to pass thru because the sun is lower compared to the horizon.A simplified explanation is it works similar to a louvered window blind that is partially opened.Because of the complexity of the formulas needed I would contact your local utility company about a free energy audit.If the windows are dual paned now it may be possible to just have the film applied to your current windows.