The energy performance of a water-in-glass evacuated-tube solar collector (WGETsc) and U-pipe evacuated-tube solar collector (UpETsc) is compared. The necessity and effectiveness of modeling WGETsc including thermal mass is validated by experimentation. A comparison of the thermal performance of systems installed with WGETsc and UpETsc having the same efficiency curve was made. The average thermal efficiency of WGETsc is less than that of UpETsc; WGETsc storage energy 25-35% is less than that of UpETsc because of fluid thermal mass influence for flow rates 10-70 kg/h m(2). If fluid thermal mass is neglected then the useful energy output will be overpredicted in numerical simulations. Moreover, the flow rate may also affect system thermal performance. The UpETsc has small optimization flow rate range of 20-40 kg/h m(2) compared with 20-60 kg/h m(2) for WGETsc. Finally, the optimal flow rate for maximum useful energy is determined by meteorological conditions such as solar radiation and outdoor temperature, the pump control strategy and even the thermal performance of the collector. (C) 2013 Elsevier Ltd. All rights reserved.