Renewable energy sources (RES) and their transformation technologies become more attractive in building sector as energy demand and emissions to the environment increase. Along with renewable energy transformation technologies, the importance of energy storage also arises as these RES are characterized by volatility and energy mismatch over time. For this reason, engineering systems which use RES are becoming increasingly dynamic. This study examines ongoing processes in a vertical cylindrical storage tank designed to cover domestic hot water demand in a building. Simultaneous dynamic mode of operation when the energy is being charged and at the same time discharged from the storage tank is analysed. Experiments are performed and stratification number describing thermal stratification is calculated. The obtained data are used for the development and validation of the numerical model. Using Ansys Fluent software, the influence of different discharge rate on operation mode of the storage tank is investigated. Graphical and numerical interpretation of results is performed.