The authors established a technique for measuring heating or cooling output from a heat pump air conditioning system by applying the refrigerant enthalpy method. In this paper, advantages of the technique and estimation process of the heating or cooling output are introduced. In addition, field experiment with a water source heat pump unit was carried out to validate the established technique. In the experiment, the refrigerant saturation temperatures at inlet and outlet of compressor and compressor frequency, which are estimated in the technique, are firstly verified by comparing to measured values. As the result of experiments, it is confirmed that the established technique have possibility of omitting foundation of the pressure sensors and the measurement of compressor's frequency in the technique. As the another results of experiments, the error of estimation values of heat quantity between the established technique and the conventional one is less than 5 % for cooling and 7 % for heating. Therefore, the technique for heat quantity measuring that the authors established has the potential to become the new measuring method for the heat pump air conditioning system.