I have been doing a bit of searching for ways to help my mom cut her $500/month (she has multiple houses on her property) utility bill. It looks like one of the easiest things to do is to build a few Thermosyphon Solar Water Heaters for her houses. She lives in Hawaii, so she gets really good sun most of the year. I've seen quite a few DIY sites, this one seems to give quite a lot of info.http://homepower.com/view/?file=HP58_pg30_Bocci
I am planning on doing a few yard mounts, basically a little lean-to with the slanted side being the solar thermal collector and the box housing the tank. Since she a collection of small 200 sq/ft buildings next to each other as her "house" with the bathroom already being a separate building, I was also thinking of making a shower house for her. Something less than 200 sq/ft, that has the roof slope to the ground on one side at about 30 degrees. That side will have the solar collector, under it will be the tank, and the rest of the room will be a big shower room. In Maui, structures under 200 sq/ft don't need building permits.
So, I've been mulling this over in my head and searching a lot. Thinking that one day when I have enough for the materials, I will put one together to test some things out. Then, today on the way home from the post office, I pass a house that has 2 huge glass panels leaning on the tree out by the curb. I check with the home owner and get his blessings to haul them away. They look like they are probably from a sliding glass door or something. I have read that there are other types of glass that are better for efficiency, but you can't get much more efficient than free.
I had figured that the glass and the copper pipe would be the most expensive parts of the build, so this is going to be a huge aid in getting them built on the cheap. So now I'm inspired to get the rest of it built to start doing some DIY testing.