Thermal storage systems with phase change materials are predominantly designed, analysed and optimised through numerical modelling. An alternative simplified method is being proposed for the characterisation of these phase change thermal storage systems. The method is based on the effectiveness-number of transfer units (E-NTUs) technique. A simplified mathematical representation has been analytically developed using the E-NTU technique for a cylindrical tank filled with phase change material (PCM), with heat transfer fluid flowing through tubes inside the tank. Experiments have been carried out on a cylindrical tank filled with PCM and with one, two and four coils of tubes to validate the technique. Experimental results for the systems with a high heat transfer area compare well with those calculated from the model. The results show that this technique can readily be used as a design tool for sizing and optimising a thermal storage unit with phase change materials. From this study, it may be concluded that the model based on the E-NTU technique can accurately predict the average heat exchange effectiveness of the thermal storage system with a high heat transfer surface area during charging and discharging. (C) 2011 Elsevier Ltd. All rights reserved.