Solar water heating systems use the sun to warm domestic hot water. Any additional hot water is supplied by the conventional boiler or immersion heater.
Solar panels on your roof collect heat from the sun and use it to warm water which is stored in a hot water cylinder.
A boiler or immersion heater is then able to raise the temperature of the water until it reaches the temperature set by the cylinder's thermostat. (This should be set at 60 degrees centigrade.)
In the UK, solar hot water is quite efficient during the summer months, but even in winter, the system raises the incoming water temperature by several degrees.
At the moment, solar water heating is not very cost efficient as 4 sq.metres of solar water heater panels will cost around £3000 (how?) to install and only save around £700 in energy bills over the 10 year warranty period.
The cost of systems needs to be radically reduced by the manufacturer before they are economically viable.