This paper deals with the numerical analysis of the energy performance of HVAC systems for heating and cooling, based on a reversible electrical air-source heat pump. The aim of this study is to highlight in which way the energy consumption of these systems is influenced by the heat pump modulating capability, the heat pump sizing and the climate. A numerical model based on the bin method and the energy signature procedure, able to take into account the performance of the heat pump at partial load, has been followed in order to calculate the seasonal and annual performance of the HVAC system coupled to a typical office building, located in three different European locations (Frankfurt, Istanbul, Lisbon). Different kinds of heat pumps (mono-compressor on-off, multi-compressor and inverter-driven heat pumps) have been considered. The numerical results presented in this paper point out that the heat pump sizing strongly affects seasonal and annual HVAC energy consumption: the ratio between the full thermal capacity of the heat pump and the building peak load strongly influences the energy performance of the system. It has been demonstrated that, depending on the season, the seasonal performance of the above-mentioned kinds of heat pumps are influenced in a different way by their sizing.