A numerical one-dimensional, finite-difference, semi-implicit model of liquid stratification in thermal storage tanks has been developed. The model can be used for simulating charging and discharging of the thermal storage tank, with simple handling of inflow-outflow boundary conditions. The convective part of the numerical model is treated explicitly using a stable second-order time-stepping scheme to minimize dispersion errors, while diffusion is treated implicitly. The scheme proved to be stable and second-order accurate spatially and temporally. Comparisons are made with experimental data. A modification is carried out on the model to incorporate actual turbulence mixing that occurs in the inflow region of any storage tank due to the effect of the diffuser design. A proper spatially dependent diffusion term is added to modify the energy model to account for turbulent mixing. The predictions are in agreement with experimental data. The effect of inter-mixing on the reduction of storage tank efficiency has been evaluated. The complete model can be used in applications that involve a component for energy storage, such as in solar energy systems and in air-conditioning systems with load management.