Your solar power supply is supplied with water from below. There is a water inlet port, the electric heater port you mentioned, and a return port for the water to flow back when full. This port will connect the other end of the return hose to the bathroom or toilet in your home. When the solar water is full, the excess water will flow towards you along the return pipe. At this time, you can turn off the water supply. valve.
The fact that you have not connected the return pipe means that your solar system has a meter and an automatic water supply function.
The one on the side of the water tank is mainly used to connect the atmosphere inside the insulation barrel with the outside atmosphere. If the insulated barrel is sealed, a lot of water vapor will be generated when the water is heated, and the pressure inside will be very high, causing the insulated barrel to burst.