a water heating device which includes a thermally insulated water storage tank having a side wall and being configured to receive drinking water , a water inlet as an inlet for drinking water , a water outlet as an outlet for hot water , and a heating unit arranged inside the water storage tank adapted to heat water inside the water storage tank . the heating unit is inclined with respect to the side wall of the water storage tank , in particular at an angle other than 90 ° with respect to the side wall of the water tank .