Goes to show all those kitchen scraps you throw away is actually costing you more money that you think!
Did you know that when compost breaks down it produces heat? With this heat you can warm up water in the pipes that run through the compost. Obviously the more pipe and the more compost you have the more warm water you get. This article and attached PDF design guide is a fairly large scale project, but could theoretically be scaled down to fit your needs.
The Design Guide states that, “A 30-40 yard mound with 900 feet of water line inside should be able convert 45-degree well-water to 110-140 degree water with a sustainable flow-rate between 1 and 4 liters per minute.” Now that’s efficient! If you are lucky enough to have that much compost material handy, then you are on your way to having a sustainable hot water supply even off the grid.