It is rare to see systems rated below SEER 9 in the United States because aging, existing units are being replaced with new, higher efficiency units. The United States requires that residential systems manufactured after 2005 have a minimum SEER rating of 13. ENERGY STAR qualified Central Air Conditioners must have a SEER of at least 14.5. Window units are exempt from this law so their SEERs are still around 10.