Abstract: The load of a compressor of a refrigeration unit is determined and from this determination a standard condenser head pressure is determined. This standard condenser head pressure is compared with measured condenser head pressure. If there is a null comparison, then the air conditioner is operating properly. If the condenser head pressure as measured varies beyond a band from the predetermined condenser head pressure for the load, it is symptomatic of less than optimum operating efficiency. Compressor load is determined as a function of the condenser cooling fluid temperature at the inlet and outlet from the condenser, or one of these temperatures and the power required by the compressor.