This article aims to investigate the effect of inertia on the outlet temperature of an evacuated tube solar collector. The experiments were conducted during a 24h period in Minquin County (latitude 38034’N, Longitude 10303’E). An analysis was made to determine whether the evacuated tube solar collectors’ fluid thermal inertia impacts the water storage tank temperature. The results showed differences between the useful energy delivered to the water storage tank in the case with mass flow rate adjusted depending on the incident solar radiation for the entire two days in which experiments were conducted. Two days of cloud and sunshine were considered. An indirect optimal control technique refers to the impacts of the inertia of water on the outlet collector temperature, the on/off control of the pump, and the useful heat gain from the storage tank. During days with clear sky, the average solar radiation, mass flow rate, outlet, and tank temperature were 640.02W/m2, 0.06kg/s, 290C, and 44.240C, respectively. For cloudy day was 449W/m2, 0.02kg/s, 270C and 420C respectively. The control system is provided to run the heating circulating pump after rest for several minutes to facilitate the exhaust system, then continues to run for a cycle. The DTP,ON and DTP, OFF range from 30C to 100C and from 0.20C to 1.50C, respectively.