In operating a series solar heat pump system, a common practice is to use direct solar heating instead of the heat pump whenever the temperature of the solar-heated water exceeds a certain value called the switch-over temperature. The switch-over temperature setting has a significant effect on the system’s energy consumption. It is shown in this analytical study that the ideal switch-over temperature increases with the building load, but that it has a maximum value determined completely by the characteristics of the water-to-air heat pump. If, because of its controller, a series solar heat pump system is to have a fixed switch-over temperature independent of the building load, then this temperature should not exceed the ideal switch-over temperature corresponding to the building design load if the energy consumption of the system is to be minimized.