Phase change materials (PCM) have been repeatedly proposed for use in solar domestic hot water (DHW) systems. PCM storage designs have been proposed, but no detailed evaluation has been made of the actual contribution of the PCM to the total heat storage tinder typical end-use conditions. In this work annual simulations were done to compare the performance of a storage tank with PCM to a standard tank without PCM. A model was constructed to describe the heat storage tank with and without PCM, the collector, pump, controller and auxiliary heater. Realistic environmental conditions and typical end-user requirements were imposed. Annual simulations were carried Out for different sites. load profiles, different PCM volume fractions, and different kinds of PCM The results of all simulation scenarios indicate that. contrary to expectations, the use of PCM in the storage tank does not yield a significant benefit in energy provided to the end-user. The main reason for this undesirable effect is found to be increased heat losses during nighttime due to reheating of the water by the PCM (c) 2008 Elsevier Ltd. All rights reserved.