Abstract: A method and apparatus for measuring and monitoring critical parameters of a chiller, such as the chiller efficiency. Specifically, temperature and pressure sensors are disposed in the inlet and outlet lines of a condenser and chiller unit. A sensor is also placed on the compressor motor for determining the amount of power being expended by the motor. The actual flow rate of the chilled water entering and leaving the chiller unit is calculated based upon the sensor inputs. The actual kilowatts per unit ton rating for the evaporator is determined. The design kilowatts per unit ton rating for the evaporator is corrected to take into account any variation between the actual motor voltage and design motor voltage, and the total temperature deviation. The chiller efficiency is then calculated based on these two kilowatt per ton ratings.