This study aims to investigate the optimal shell-to-tube radius ratio in a vertical shell-and-tube latent heat thermal energy storage system with phase change material packed in the annulus and heat transfer fluid circulating in the central tube. A conjugate thermodynamic model is developed and validated, and then applied to investigate two series of system configurations: varying the phase change material shell radius at a fixed heat transfer fluid tube radius and varying the heat transfer fluid tube radius at a fixed shell radius. The numerical investigation compares and evaluates energy storage/retrieval density, energy storage/retrieval rate, and total stored/retrieved energy capacity under different shell-to-tube radius ratios. The results show that the thermal behaviour in both series of units is very similar. The optimal radius ratio slightly increases as the total charging/ discharging time increases. The unit height has a gentle effect on the optimal radius ratio in the charging process and a negligible influence in the discharging process. By balancing the energy storage/retrieval density, energy storage/retrieval rate, and storage/retrieval capacity in both charging and discharging processes, the optimal shell-to-tube radius ratio is found to be around 5 for both series of configurations at the studied total charging/ discharging times.