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Abstract Thermal storage technology is an important approach to improve the efficiency of solar energy utilisation. To analyse the thermal stratification in a hot water storage tank at an initial temperature of 80 °C and at an inlet temperature of 5 °C, this study thoroughly analysed the impact of the position of PCM balls on the thermal stratification for various flow rates (1, 3, 5, 7, and 9 L/min) under different position. Performance parameters, including the Richardson number, MIX number, and exergy efficiency, were involved in the evaluation. The study was further extended to explore the applicability of Fill Efficiency (FE) as a performance parameter of thermal stratification within a storage tank. The experimental results indicated that, for the same inlet flow rate, an improved thermal stratification of the tank was achieved as the positions of the balls became closer are to the inlet, and the balls with a small diameter also showed remarkable potential for improving the thermal stratification of the water tank. When the flow rate increased, the cold and hot water mixing intensified, and the thermocline thickness in the tank increased. Thus, the thermal stratification weakened. Furthermore, when the water-release process progressed, the mixing of the hot and cold water in the water tank tended to be stable, and formed a stable thermocline. Finally, when the inlet flow rate was less than 7 L/min, the thermal stratification of the water tank exceeded that of the PCM water tank, while for an inlet flow rate in excess of 7 L/min, the thermal stratification of the PCM water tanks was superior to that of the water tank in the case when the balls was on the fourth layer. Highlights • A thermal storage tank containing sodium acetate trihydrate with phase change temperatures of 48 °C and 58 °C are developed. • Impacts of the positions and diameters of PCM balls on the thermal stratification with various flow rates are analysed. • The thermal stratification performances in hot water tank are compared. • Fill efficiency is used as a measurement index to determine the performance of a thermal storage tank. [ABSTRACT FROM AUTHOR]