The thermal performance of an integral compact solar water heater is analysed numerically and compared with experimental data. In the proposed system the collector and the water tank are built into one unit with water flow caused by thermosyphonic force. Two geometries of the proposed system are investigated. While the proposed system presents a new concept for solar water heater, the efficiency of such systems are shown to be larger than conventional ones. The numerical scheme used does not make any a priori assumption concerning temperature profiles.