The efficiency of the various technical building subsystems − such as emission, control, distribution, storage, and generation − significantly impacts overall building energy performance. Accurate calculation methods are essential for assessing subsystem heat losses and efficiencies. However, modelling these components while considering the interactions with each other, the building envelope and users is a complex issue, limiting the widespread use of highly accurate procedures. Therefore, numerical approaches that balance simplicity and accuracy in the modelling of the subsystems are needed. Following the publication of Mandate M/480 EN, efforts have been made to improve the assessment of heating and cooling system performance. Nevertheless, several procedures still require improvements to maximize the efficacy of energy assessment. This work specifically focuses on the analysis of thermal storage systems. Current methods often oversimplify heat loss complexities, by simplifying the modelling of the water convective motes in the storage tank. As a result, simplified procedures show discrepancies compared to detailed calculations, highlighting the need for validated approaches. This research analyses simplified calculation methods for assessing thermal storage performance, identifying key parameters that influence energy losses, and aiming for a comprehensive validation. The study includes a comparative analysis of different simplified and detailed methods applied to a residential case study.