Before an air conditioning system is installed in a home, the size of the home must be matched with the size of the coils.  This will ensure that the proper BTU cooling requirements are met.  The condenser coil (located outside the home) and the evaporator coil (located in the duct work directly above the furnace) are designed to provide efficient heat transfer to cool a particular area.  Over time, the coil's capacity to cool efficiently is depreciated by airborne contaminates clogging the coil fins.  The result is inefficient heat transfer, a decrease in airflow, and a concern for indoor air quality.

Dirty coils increase the amperage and the amount of time needed for the system to reach the desired indoor climate.  HVAC systems with dirty coils can consume 35-40% more energy than a system with clean coils.

Consider a typical cooling season from May 15th to September 15th.  This is equal to about 1600 hours of operating over 125 days (12.8 hours per day).  A typical 4 ton residential cooling system would cost about $704 to operate for 1600 hours with clean coils.  When the coils get dirty, that same system would have an operating cost of $964.48.  This is a 37% increase on your utility bill.