An investigation into characterising and optimising the useful latent energy that can be stored within a tube-in-tank phase change thermal energy storage system has been conducted, with particular reference to off peak thermal storage applications. This process involved considering the thermal resistance during charging and discharging as well as the amount of phase change material (PCM) that can be physically stored within a storage system with tubes in a storage tank for a night time cooling system using a cooling tower. The thermal resistance was investigated through the use of the heat exchange effectiveness of the PCM system which was studied using a validated ɛ-NTU model. An energy storage density coefficient of a PCM system was determined and the impact of the tube length and the mass flow rate supplied to the system was investigated. This storage coefficient was optimised delivering a true energy storage density of 62.9% and 82% of the latent energy density of the PCM. This parameter can be directly compared to sensible storage systems and it was found that coil-in-tank systems can achieve a useful storage density of more than 20 times that of sensible storage systems. Refereed/Peer-reviewed