This study presents a theoretical and experimental analysis of the thermal performance of a solar water heater prototype with an internal exchanger using a thermosiphon system. The heat exchanger made of a rolled copper tube is placed diagonally in the storage tank so that the hot fluid crosses a significant mass of stored water. The results focus mainly on the levels of the heat fluxes, temperatures recorded, mass flow rate and efficiency of the collector. During the main insulation period, one obtains satisfactory qualitative and quantitative agreement between the experimental and theoretical results of mass flow rate and temperatures. Those indicate heat fluxes whose 2 peak reaches 989 W/m(2), collector outlet water temperature levels of more than 85.5 degrees C and a collector thermal effectiveness around 58%. (C) 2008 Elsevier Ltd. All rights reserved.