A solar controller is an electronic device that controls the circulating pump in a solar hot water system to harvest as much heat as possible from the solar panels and protect the system from overheating. The basic job of the controller is to turn the circulating pump on when there is heat available in the panels, moving the working fluid through the panels to the heat exchanger at the thermal store. Heat is available whenever the temperature of the solar panel is greater than the temperature of the water in the heat exchanger. Overheat protection is achieved by turning the pump off when the store reaches its maximum temperature and sometimes cooling the store by turning the pump on when the store is hotter than the panels. Most commercial controllers display the temperature of the hot water in the store and provide general status information about the system, including overall energy production. Components The simplest solar controller circuit uses a comparator with two temperature inputs, one at the solar panel and one at the thermal store's heat exchanger, and an output to control the pump. Commercial controllers use a microprocessor usually with a LCD display and simple user interface with a few pushbuttons. Power for the controller and the pump can come from a mains electric supply or from a photovoltaic (PV) module. Function The controller's main function is to switch the circulating pump on or off. The pump is usually switched on when the solar panel is hotter than the water in the store's heat exchanger and off when the panel is colder. Switching the pump on transfers the heat in the panel to the store. Switching it off when the panels cool prevents a reversal of the process and loss of heat from the store. The controller measures and compares the temperatures in the panel and the heat exchanger every few seconds. Commercial controllers do not turn on the pump until the difference in temperature between the panels and the water in the heat exchanger is sufficient to provide significantly more energy than is consumed by the pump. This temperature difference is called the on differential (usually 4–15 °C. They turn off the pump when the panels no longer are hot enough to provide significant heat to the store (the off differential). The wider the difference between these differentials, the fewer pump on-off cycles will take place. These