The objective of this research is to determine optimal control variables of a collector pump placed on a collector loop to improve the performance of a solar domestic hot water system. A mathematical model of the system is developed to predict its operating performance under real weather conditions at Jeju Island, South Korea. The optimum control variables of the collector loop are investigated based on the relationship between the useful heat gain of the solar collector and the electricity consumption of the collector pump. In addition, the effects of various parameters such as solar collector area, initial water temperature, and volume of storage tank are analyzed. The results of the simulation showed that the optimum variable flow rate was determined at m = 0.05 Delta TA(c)/60 (kg/s) (K-nu = 0.05). At this value, the useful heat gain of the solar collector Q(u), increased by about 1.54% while the electricity consumption of the collector pump E-p sharply decreased to 65.6% when the constant flow was replaced by the variable flow for the collector loop. Furthermore, the system performance is significantly affected by the change of initial water temperature and volume of the storage tank, as well as the collector area. (C) 2013 Elsevier Ltd. All rights reserved.