All solar thermal systems need electric pumps to operate, except thermosiphon and ICS setups. However, you can't just plug the pump in and leave it running all the time. If you did, it would reverse the energy gathering process, dispersing stored heat through cold solar collectors into the night. The installation needs a control system to switch the pump on when there is heat to be gathered and off when there isn't.

Most AC-powered modern systems use differential temperature controllers for this purpose. The controller plugs in to the standard 120-volt supply (or another voltage, depending on the country) and, at its most basic level, monitors two sensors:

• Sensor 1 sits on the heat storage
• Sensor 2 sits on the collector array

When the temperature at Sensor 1 is lower than the temperature at Sensor 2 (i.e. the collectors are hotter than the storage), the controller flips a relay and switches on the pump. Solar fluid circulates, carrying heat to the storage until the two sensors measure the same (or nearly the same) temperature.

When the temperature at Sensor 2 is the same as or lower than that at Sensor 1 (i.e. the storage has reached the same temperature as the collectors or there is not enough heat to collect), the pump switches off.

Most modern differential temperature controllers are microprocessor-based and run on low voltages, so they use a transformer to step down the utility's supply. The pumps in these systems are usually 120V (or appropriate voltage) models, so the power supply does not need adjusting.

Of course, modern controllers are more complex than a simple on/off switch. Most come with a high-limit function to switch off the pump when the storage reaches a preset temperature, even if the collectors are still hotter; this avoids overheating the storage. While this is an excellent option in a drainback installation, a pressurized system really needs to keep operating whenever there is solar energy available. If the solar fluid lies idle, it will deteriorate.

For pressurized systems, a controller with a third option – a shunt load switch – is preferable. In this setup, the high limit becomes a redirection: the main circuit keeps operating and the pump keeps pumping, but the hot solar fluid is directed to a shunt load where it is dissipated or used without overheating the storage.

There's also a fourth option included on some controllers, specifically for flooded systems. This is "recirculation mode", which turns the pump on when the temperature at the collector array nears freezing point. The pump circulates warm water from the storage medium to the collectors, to stop them freezing and getting damaged.

When you shop for a controller, make sure you choose one with features appropriate to your system type. And make sure that you ground it properly when you install the system: the single biggest cause of system failure is the controller being hit by a power spike (e.g. from lightning) and shorting out.