This article experimentally and theoretically investigates a two-phase thermosyphon solar water heater. The performance of this innovative solar water heater at different solar radiation intensities and tilt angles are experimentally discussed. The results show the best charge efficiency of the system is 82%, which is higher than the conventional solar water heaters. The theoretical model is also developed using the thermal resistance-capacitor method. The simulation predictions agree well with the experimental data within an average error deviation of +/- 6%. Two methods for improving the performance of this heater, double fin tubes and nano particle, are proposed. The results show that charge efficiencies can increase 3% and 4%, respectively, according to the theoretical model. (C) 2010 Elsevier Ltd. All rights reserved.