Heat pump water heaters have been increasingly used to promote energy savings in residential buildings. Nonetheless, their performance may vary significantly due to different site-specific characteristics and technical specifications. To examine this issue, this research study sought to identify key performance parameters for air source heat pump water heaters (ASHPWH) in residential buildings in Australia by using a sensitivity analysis through theoretical models calibrated with empirical measurements. ASHPWH were analysed taking into account their energy performance (i.e. annual energy consumption and energy intensity) and level of service (i.e. compliance with recommended hot water temperatures). The assessment encompassed sensitivity analyses for key parameters related to the technical specification of ASHPWH (i.e. coefficients of performance, water heating capacities, hot water storage tank insulation, size and set-point temperature) and site-specific characteristics (i.e. weather conditions, cold water supply temperatures, hot water set-point temperatures and consumption patterns, and the time-distribution of energy according to the electricity tariffs). The findings indicate that the energy performance and the level of service of ASHPWH are mostly influenced by their coefficient of performance and site-specific electricity tariff, respectively. Therefore, careful consideration of ASHPWH technical specifications based on site-specific characteristics should underpin any framework for energy efficiency programmes.