At low ambient temperatures, the heating capacity and coefficient of performance of a single stage vapour compression heat pump cycle is significantly getting reduced. The two stage cascade heat pump cycle operating with two different refrigerants provides a sustainable solution to lift the condenser temperature above 343 K. In this work, a numerical simulation model was developed for predicting the performance of a solar collector and two stage cascade heat pump combi water heating systems under Kazakhstan climatic conditions. The numerical simulation was performed for winter climatic conditions using nine refrigerant pairs such as, R32/R290, R32/R1234yf, R32/R134a, R410A/R290, R410A/R1234yf, R410A/R134a, R744/R290, R744/R1234yf and R744/R134a. The influences of solar irradiation, ambient temperature and condenser temperature are discussed. The solar collector and two stage heat pump combi heating system has about 30% energy savings when compared to the conventional two stage cascade heat pump without integration solar collectors. The R32/R290 refrigerant pair is having maximum coefficient of performance of 2.4 at 323 K condensing temperature and 263 K evaporating temperature. The refrigerant R744/R290 pair is identified as an environment friendly sustainable option in terms of its global warming impact for two stage cascade heat pump applications. (C) 2019 Published by Elsevier Ltd.