A welcome side effect: The heat transfer to the solar liquid automatically cools the modules as the heat is deducted to the warm water reservoir. This lets the temperature of the photovoltaic cells decline. Due to this process the electric energy output increases because cool modules do have a higher efficiency factor than warm modules. Cooling the modules therefore pays off in two ways: The production of sun heated water simultaneously increases the electric efficiency factor of the solar plant. How much it rises due to the cooling effect depends on how much of the heat energy in Summer is taken...