Thermal performance characterisation of thermal storage systems employing phase change materials (PCMs) has been carried out predominantly through numerical modelling. These models do not provide a direct representation of the storage system, and cannot readily be used to design a thermal storage unit (TSU) for a particular application to meet the performance specification. Furthermore, limited consideration is given to exergy efficiency of the PCM system being studied. The phase change profiles identified in detailed simulation of PCM encapsulated in flat containers, have been employed to analytically formulate the TSU effectiveness using the familiar effective-NTU (-NTU) approach. A one and two dimension formulation were developed with respect to the phase change fraction. This single parameter function can be used for sizing a TSU, by determining the redundant amount of PCM defined by the minimum effectiveness during discharging. Furthermore by optimising design parameters, exergy losses can be minimised by maximising the effectiveness over the full charging - discharging cycle. The study demonstrated that for air based systems phase change can be represented in one-dimension, however for liquid based systems two-dimensional phase change needs to be considered. © 2012 Elsevier Ltd. All rights reserved.