Thermal storage with latent heat of phase change materials (PCM) is a desirable component in solar thermal power plants based on steam cycles, to provide heat for evaporation at nearly constant temperature, leading to stable plant operation and longer operation hours. This paper analyzes the relation between stored heat discharge rate, plant operation mode, and the power plant overall performance. The storage and plant models are simple and easily solved, yet they describe major performance characteristics observed in experiments. We show that the sliding pressure mode with constant heat extraction, currently preferred for operation from storage, does not provide constant electrical power output. A variant of sliding pressure, accounting for the variation of the thermodynamic cycle efficiency, can achieve nearly constant electrical power output. Moreover, the model shows that constant power operation can be achieved even for natural PCM without heat transfer enhancement, potentially reducing the complexity and cost of the storage system. (C) 2013 Elsevier Ltd. All rights reserved.