I found this little bit of info to be an interesting quote from Wiki;
Residential solar thermal installations can be subdivided into two kinds of systems: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50°C. Hence, hot water is always available.
Seems to me that you better be staying in your home a long time before it pays off.