Abstract: A method and instrument is provided which allows quick and accurate measurement of the coefficient of performance of an installed electrically powered heat pump including auxiliary resistance heaters. Temperature sensitive resistors are placed in the return and supply air ducts to measure the temperature increase of the air across the refrigerant and resistive heating elements of the system. The voltages across the resistors which are directly proportional to the respective duct temperatures are applied to the inputs of a differential amplifier so that its output voltage is proportional to the temperature difference across the unit. A voltage-to-frequency converter connected to the output of the differential amplifier converts the voltage signal to a proportional frequency signal. A digital watt meter is used to measure the power to the unit and produces a signal having a frequency proportional to the input power.