We have 3m2 solar Panels which are of the vacuum type, and are used to heat our hot water. Any excess is automatically transferred into the large Buffer Tank used for our underfloor heating.

I have built a small controller from an Arduino which checks the Solar Tank (hot water) temperature during the night to see if it has fallen below a particular threshold. This will occur if there has been little or no sun and wind. The controller will then turn on the Mains Immersion Heater to heat the water on the cheap rate of electricity. It is also used to log the tank and pipe temperatures which has been useful fine tuning the system.