A thermal energy storage medium must meet the requirements of a stable storage material with high heat capacity. Heat storage based on the sensible heating of media such as water, rock and earth represent the first generation of solar energy storage subsystems and technology for their utilization is well developed. However, recently the heat storage based on the latent heat associated with a change in phase of a material offers many advantages over sensible heat storage. The most important characteristic of such a subsystem is its sufficient storage capacity. The PCM (phase change material) behavior is visualized by constructing an idealized model thermal capacitor subjected to simulated solar system environmental conditions which include thermal cycling utilizing the latent heat of paraffin for heating and cooling. The proposed model of the capacitor is of flat plate geometry consisting of two panels compartments forming the body of the capacitor containing the paraffin, welded from their inner surfaces to a thin copper one allowing the passage of the water. The whole structure was assumed to be insulated to minimize heat loss. An analysis of the model is conducted using Goodman technique . To generate data about the temperature distribution, the melt thickness, and the heat stored in the PCM under two types of conditions were generated; namely (i) constant mass flow rate tests for various water inlet temperatures and (ii) constant water inlet temperature for various mass flow rate. It was found that water outlet temperature increases with time until it becomes nearly equals to the inlet temperature. Increasing the mass flow rate for a given inlet temperature, decreases the time required for outlet temperature to reach a given value. Increasing inlet temperature for a given mass flow rate gives a very rapid decrease in the time required for the outlet water temperature to reach a given value. Instantaneous rate of heat storage was determined from the inlet-to- exit temperature differential and measured flow rate. This rate was then integrated numerically to determine the cumulative total energy stored as a function of time. It was found that the instantaneous rate of heat storage decreases till reaching a nearly constant value. The total or cumulative heat storage as a function of time, showed a nearly linear trend in the mid-range time, and it increased with increasing inlet temperature.