The invention discloses a self-cleaning water tank and application of the self-cleaning water tank to an electric water heater and a solar water heater. The self-cleaning water tank comprises a water tank body, a circulation cleaning tank and an ultrasonic assembly. The water tank body is provided with a water inlet and a water outlet. The bottom of the water tank body downwards inclines to form a dirt outlet. A liquid inlet is formed in the top of the water tank body. A filter is arranged in the circulation cleaning tank so that the circulation cleaning tank can be divided into a deposition tank and a filter liquor tank. The ultrasonic assembly comprises an installing frame arranged in the water tank body, and a plurality of ultrasonic transducers arranged on the installing frame. A liquid inlet pipe communicates with the liquid inlet and the filter liquor tank, a dirt discharging pipe communicates with the dirt outlet and the deposition tank, a cleaning fluid circular flowing path is formed, and the end, stretching into the water tank body, of the liquid inlet pipe is connected with a spraying device. The self-cleaning water tank has the beneficial effects that the dismounting-free automatic cleaning function is achieved for water storage type water heater cleaning, convenience and safety are achieved, the cleaning cost is reduced, and the situation that incrustation breeds bacteria, and the body health is influenced is effectively prevented.