Solar domestic hot water systems are very common and are considered an efficient system for exploiting the renewable energy source of the sun, for instance national Italian regulations require the adoption of solar domestic hot water systems in new buildings. Water tanks are typical components for such systems, but have a serious drawback since they require a great amount of space in buildings. In this article, a new water tank model with embedded phase change material modules for the ESP-r code is presented in order to assess the possibility of using reduced size water tanks with additional latent storage. This article describes the numerical model used to deal with phase change phenomena and the incorporation of the method into ESP-r source code. To validate the model, the results of a test case have been compared with the literature data. A typical solar domestic hot water system has been simulated in order to study the impact of the phase change material modules inside tanks on the energy performance of the system. The results are reported using a global parameter that represents the fractional primary energy saved using the solar system. Water temperature plots with and without phase change material have been reported as well.