In this paper, the performance of a triangular built-in-storage solar water heater under winter conditions has been studied. The triangular cross-section of the system results in higher solar gain and enhances the natural convection leading to a higher water temperature. The forward finite difference technique is employed and initial conditions arc derived from the assumption that the water temperature and the temperature at each node are in equilibrium with the ambient temperature. The performance of the system is analyzed under no flow, constant flow rate and intermittent flow rate conditions. The effect of the night insulation cover is also analyzed. A comparative study of the triangular water heater with a rectangular one is also made. The effect of the tilt angle on the water heater is also discussed. The efficiency of the system under various conditions is also discussed.