In this paper a numerical model for the calculation of the seasonal performance of different kind of electric air-to-water heat pumps is presented. The model is based on the procedure suggested by the European standard EN 14825 and the Italian standard UNI/TS 11300-4, which specify the guidelines for calculation of the seasonal performance of heat pumps during the heating season (SCOP), the cooling season (SEER) and for the production of domestic hot water. In order to consider the variation of outdoor conditions the developed model employs the bin method. Different procedures are proposed for the analysis of the seasonal performance of mono-compressor, multi-compressor and variable speed compressor air-to-water heat pumps. The numerical results show the influence of the effective operating mode of the different heat pumps on the SCOP value and put in evidence the impact of the design rules on the seasonal energy consumption of these devices. The analysis points out that inverter-driven and multi-compressor heat pumps with an optimized design can present higher SCOP than mono-compressor devices of about 10-15%. The study also highlights the importance of the correct sizing of the heat pump in order to obtain high seasonal efficiency and it shows that, for a fixed thermal load, inverter-driven and multi-compressor heat pumps have to be slightly oversized with respect to mono-compressor ones in order to obtain for the same building the highest SCOP values.