Replacement windows affect more than appearance. The right combination of glass, frame, fit, and installation can influence comfort, daylight, condensation, and heating or cooling demand.
Read performance labels as a system
U-factor describes heat transfer through the window. Solar heat gain coefficient describes how much solar heat passes through. Visible transmittance relates to daylight. Lower or higher is not automatically better in every climate and orientation.
Do not overlook installation
Even a well-rated unit depends on accurate measurement, water management, air sealing, insulation, and careful exterior and interior finishing.
Compare like with like
Ask for the complete unit ratings, the proposed glass package, frame material, operation style, warranty terms, and written installation scope.