Good ol’ John Canivan – guru of sun heated water systems, related the following to me:
In 1970 a friend of mine left the civilized comforts of a New Jersey home to seek out and experience the free, wild wilderness of an Adirondack hilltop in upstate New York. Jake loved the country life and swore that he’d never leave his mountain retreat. He endured the cold winters without electricity and baseboard heating, but Jake still missed running hot water. When spring came he took a 300-foot coil of black plastic tubing and connected one end to a spring high up on the mountain. He draped the remainder of the 300-foot coil on his roof and spread it out to cover as much surface area as possible. For a $50 investment and one hour’s worth of labor Jake had himself a bona fide hot water shower. He made good use of it whenever the sun was high on the mountain. I used it a few times myself. It works. The roof might look a little funny and the hot water would sometimes run out sooner than you’d like, but it did work when the sun was shining.
John has a practical tutorial on the how’s and why’s of sun heated water on his website.