The basic principles of solar thermal can be explained quickly and simply:
Solar Thermal panels are designed to be directed towards the suns natural light, this light energy is absorbed in to the fluid within the panels. This heated fluid is then pumped from the panels through pipework until it is exchanged in to your hot water cylinder using a coil within the cylinder.
The cooled fluid is then pumped back to the panel and reheated and the process starts again.
Other than the small amount of electricity used to power the pump and controller, there is next to no energy required to run solar thermal other than the sun.