An analysis of transient, two-dimensional, mixed convection and thermal stratification in rectangular hot water storage tanks is presented. The governing equations together with inflow and outflow boundary conditions are written for laminar mixed convection flow based on Boussinesq approximations and conjugate heat transfer. The equations are solved numerically using a finite volume based computational code and the results are obtained for varying aspect ratios from 1 to 4 in the Richardson number range of 105 to 10 8 in the dynamic charging mode. The transient temperature profiles in the bulk fluid reveal mixing at the top with increasing Richardson numbers during charging process. Mix Number and Charging efficiency is used to quantify the system performance in the dynamic mode of operation. The results indicate mixing has significant influence on the charging efficiency. The charging efficiency increases Richardson number for constant Peclet number and Fourier number, in addition with increasing Peclet number and Fourier number, charging efficiency increases.