Thermal stratification is an important factor increasing the efficiency of hot water storage tanks used for thermal energy storage. In this study, the effects of the position of a rectangular hot water tank and the aspect ratio on the thermal stratification were investigated numerically. In this direction, a numerical analysis was carried out for the different angle of the water tank with the horizontal axis (α) and the different aspect ratio of the water tank (D/H). Numerical analyses were performed at 0–60° range of angle values and 0.5–1 range of aspect ratio values. The results obtained from the analyses indicate that positioning of the hot water tank at different values from α = 0° provides thermal stratification enhancement. Reducing the aspect ratio of the tank contributes to the thermal stratification only for the angle of α = 0° significantly. In other angle values, the influence of different aspect ratios on the thermal stratification is negligible level. The highest temperature difference in the hot water tank occurred for the angle of α = 45° and aspect ratio of D/H = 0.5.