Numerical modelling is commonly used to design, analyse and optimise tube-in-tank phase change thermal energy storage systems with fins. A new simplified two dimensional mathematical model, based on the effectiveness-number of transfer units technique, has been developed to characterise tube-in-tank phase change material systems, with radial round fins. The model applies an empirically derived P factor which defines the proportion of the heat flow which is parallel and isothermal. This P factor was determined using a validated computational fluid dynamics model. This method can be used to design and optimise tube-in-tank salt based phase change thermal storage units with finned tubes.