This study presents the development of a model to characterise the thermal performance of brick geometries in a packed bed thermal energy storage (TES) system. The research focuses on optimising heat transfer between ceramic clay-based bricks and molten salt heat transfer fluid (HTF) in a packed bed configuration. A single-brick heat transfer model is developed, considering factors such as void fraction, channel arrangement, and brick shape. The model employs an extended lumped capacitance method to analyse heat transfer coefficients and Biot numbers for various geometries. A case study compares the thermal performance of two brick designs, demonstrating the model’s ability to estimate cooling times and heat transfer rates. The flat plate design performs better with larger heat transfer and lower pressure drop. The model provides valuable insights for early-stage brick design in packed bed TES systems. This research contributes to optimising packed bed TES systems for improved thermal performance and efficiency in solar thermal energy applications.