"The cooling delivered and the efficiency of your air conditioning systems should not decrease over time unless the outside coil suffers from corrosion or the inside coil gets really dirty. So an air conditioner that is 7 years old should be just as good as it ever was as long as it has the right amount of pure refrigerant in it. The Federal minimum standard became SEER 10 in 1992 and changed to SEER 13 in 2006. So if you have a unit built after 2006 there is little reason to change it based on economics? When you buy an air conditioner make sure: You have a contractor test the duct leakage and seal the ducts using mastic (NOT DUCT TAPE). That may very well save more money than replacing the air conditioner. You have the contractor replace both the outside unit AND the inside coil (called the evaporator coil). If you don’t do that you will not get the efficiency you were promised. "