i once seen a web site where someone used solar heated water to heat their home and water in a closed loop from underground to cool the house. this was set up apparently with the termostat and a car battery connected to a solar panel, his heating and air was supposed to run for 2-3 days without needing a charge running a 6 volt water pump. just thought it might be something fun to work on. im not sure how well radient heat works but if it works as well as people boast what about radient cooling? painted cement floors or tile and you got living off the grid a little more comfortable :-p
Here is a sight with hundreds of do it yourself projects involving solar heating/cooling, wind etc etc. There is lots of great info and projects along with individual accounts of successes and many ways to cut down on your electric usage. You will find several plans/constructions of solar water heating
http://www.builditsolar.com/index.htm