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A ER is an energy rating system developed for windows and sliding doors which compares the amount of energy lost through air leakage and through the glass, spacers and frames with the amount of heat gained through solar gain. It is expressed in watts per squar metre and can be a negative or positive number. A typical ER number of a single glazed window is -50, for double glazed -30, and for low-e agron filled betweeb -12 and +4.

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