An experimental and numerical study was performed on a horizontal storage tank of a thermosyphon solar hot water system to verify if a one dimensional heat flow model is suitable for prediction of the temperature gradients in the tank. The tank was instrumented with 24 thermocouples distributed in the vertical, axial and radial directions. Two types of tests were conducted on the system, namely with and without hot water removal (load). It was found that the temperature gradients in the radial and axial directions may be neglected compared to that in the vertical direction and that the average storage temperature may be calculated from the vertical distribution at the center of the tank.