Variable speed compressor is one of the most promising ways to modulate thecapacity of heat pump units. The paper describes some common obstacles in the evaluationof variable speed heat pump systems. The inconsistencies between the compared systemcomponents, the complexity in the measurement of some detailed parameters, too muchfocus on unit efficiency solely, but also not considering the effect of system dynamics aresome of the mentioned problems. Dynamic modelling and simulation of a heat pump systemtaking all system components into consideration and validation of the model based onexperiments are suggested as the solution. This paper describes a parameter estimationapproach to variable speed heat pump unit modelling that requires a relatively low demand ofmeasured input data. The outputs of the model are a number of parameters such as totalisentropic efficiency of compressor as a function of frequency, superheat temperature andsub-cooling temperature, etc. Making use of the model provides the opportunity to evaluatethe dynamics of the whole system including building and heat source in TRNSYS.