I installed a DIY solar water heating system on my house. It's a two-tank system. The solar system heats water in a 120 gal storage tank, which then feeds into a standard 40 gal gas water heater. During the summer, the solar system does all the work, and the gas heater never fires up. In the winter, the solar system gets the water up to about 90 degrees, and the gas heater does the rest of the work.

I estimate the solar system saves us almost $400/year at current gas rates.