I think it would be better to drain the water.
If the solar water heater is not used in winter, the water from the solar water heater should be drained, the valve closed and the interior plug disconnected to keep the exterior water lines free of water and not frozen. Prevents water pipes from freezing and cracking due to low temperatures and lack of water circulation.
If the solar water heater is filled with water in winter, when the temperature is too low, the water in the pipes will easily freeze and freeze the pipes. In severe cases, the solar water heater will freeze and. crack.