One of the most common mistakes made when installing a central air conditioner is assuming that the higher the SEER, or Seasonal Energy Efficiency Ratio (this is the effeciency rating posted on the outdoor condensor unit), the higher the efficiency of the system. Not so fast. Yes, the higher the efficiency of the air conditioner condensor the better, however a factor of primary importance is the evaporator coil (indoor coil). Evaporator coils have huge efficiency differences between them, and the true SEER of your air conditioning system needs to be calculated in terms of the system as a whole. The outdoor unit is only one piece in the system.
Consider the following performance measurements.
A Lennox 14 SEER air conditioner condensor combined with a C33-48+ evaporator coil operates at a combined system efficiency of 13.0 SEER. Yet that same Lennox 14 SEER air conditioner condensor combined with a C(A,C,D,E)36C34+ evaporator coil operates at 13.5 SEER. Again, that same 14 SEER unit will only reach 14 SEER with a A(A,B)W364+ evaporator coil or something similar. SEER ratings as high as 14.5 and 15.0 can be achieved if this air conditioner condensor is used in combination with a variable speed DC blower motor.