This paper aims to improve the overall efficiency of heat pipe solar water heating (HPSWH) systems by implementing a novel variable mass flow rate technique which regulates the solar working fluid mass flow rate of the system with the solar radiation intensity. To analyse the system under real operational conditions, the residential hot water consumption pattern of Perth residents in Western Australia was used in the experiments. In addition, a nanofluid (Al2O3/DI) was fabricated and its performance as the solar working fluid was investigated to find the optimum concentration and to confirm its stability and thermo-physical properties consistency. The HPSWH system was operated during three days having similar climatic conditions using distilled water at a constant mass flow rate (Case I), the optimized nanofluid at a constant mass flow rate (Case II), and the optimized nanofluid at a variable mass flow rate (Case III). The results revealed that 0.1 wt% Sodium Dodecyl BenzeneSulfonate (SDBS) was the optimum concentration of SDBS for 0.05 wt% Al2O3/DI water nanofluid at which it exhibited the highest thermal conductivity enhancement and stability. Moreover, the transferred energy to the solar working fluid in Cases II and III were respectively 8.9% and 22.7% higher than Case I. The system had respectively 12.46% and 19.34% higher thermal efficiencies in Cases II and III compared with Case I. The exergy efficiency improvement of Cases H and III were respectively 1.58% and 2.66% compared with Case I. Overall, the results proved the significant effectiveness of the variable mass flow rate technique to improve the thermal performance of HPSWH systems.