Nothing too complicated about this project—use solar radiation, a principle as old as the sun itself, to heat water. The trick is transferring the energy from one body of water (your solar hot water preheat system) to another (your shower.)
The exact details of how you’ll rig your system will vary depending on the climate. Regions with lots of days below freezing can result in frozen pipes, so proceed with caution, but building your own preheat system is way simpler in warmer climes. Before hitting the hardware store, stock up on some back issues of Home Power magazine—a treasure trove of DIY renewable energy projects.
A simple system perfect for the southlands starts with painting a 55-gallon drum black. For bonus points, build an insulated box with a glass top around the barrel. Now, study up on plumbing and install a line between your domestic water supply, your newly minted solar hot water preheat, and your hot water heater. If the pre-heat gets the water hot enough, the hot water tank won’t even turn on. If it's not hot enough, the water heater will add heat to bring it up to temperature.
“This is a simple way to save a bunch of energy especially in hot climates. After all energy from the sun is free,” says Andy Tyson, owner of the solar design and installation firm Creative Energies.
In cold climates, extra insulation and care should be taken when designing a pre-heat system. Consult with a local solar thermal installer or take a class from a reputable institution like Solar Energy International in Carbondale, Colorado.