There are different types of solar controller detailed here. They are sometimes called "differential solar controllers".
The solar controllers are small electronic boxes that control how a solar water heating system operates. The solar controller decides when to turn on the pump (ie when the collectors on the roof are hot) and then when to turn the pump off, ie when the sun goes down or the sun is not hot enough to add more heat to your water tank.
Besides controlling the main solar pump, the RESOL controllers can do much more. They can also control a second zone such as heating zone by turning on or off a 2nd pump or control valve. All RESOL controllers come with built in Energy Metering, which allow a user to find out exactly how much energy they are saving. This is also a great way to adjust the performance of the system.
RESOL controllers set the standards in solar differential controllers. With over 2,000,000 million installations world wide.