Can be cheap, fun and very worthwhile.
It's not all that difficult to build your own Solar Batch Water Heater and the results are well worth the time and effort. Free hot water from the sun is an awesome thing, particularly when you consider the energy savings and the significant amount of money you can save on your power bills. These water heaters are also sometimes referred to as Bread Box Solar Water Heaters or even the complex sounding name of Integral Passive Solar Water Heater.
All you really need are some basic carpentry skills and tools, a tank that doesn't leak, some black paint, some shiny builders foil, a sheet of tempered glass, a pile of insulation, some timber and some sheet exterior cladding. There are lots of ways to optimize your Solar Batch Water Heater project, but really all you need is a tank with the cold water fed to the inlet in a well insulated box under glass pointed at the sun and you'll definitely get at least some hot water for free from the outlet.
The idea is you take that heated water and connect it to the inlet of your existing conventional water heater, that way when the sun doesn't shine the existing appliance will heat your water just like it always did. On the other hand when the sun is out the water being fed through the system can quite likely be hot enough that the thermostat does not switch on your electricity or gas to heat the already hot water. It's a simple reliable system and fail safe too.
That is the greatest strength of the Solar Batch Water Heater, it has zero moving parts, no controllers, no microchips, no circuitry, no pumps or other mechanical devices to fail in service. A well built Batch Heater can last for 25 to 30 years without any major problems.
I would strongly advise getting a licenced plumber involved for the plumbing connections to your batch heater but beyond that, the rest you can do in your shed or backyard in a couple of days once you have gathered the materials.
Now if you are thinking about giving this a go yourself I'd suggest at the least you read through some of my more in depth posts to get a solid grounding in the theory and practical application of Batch Solar Heaters. They can be found Here, also here and here. The first article also contains a video of yours truly describing the system fully and discussing some of the plumbing considerations to ensure you are safe.
A final word of warning, don't mess with hot water plumbing if you have no idea what's going on. Stored energy is dangerous when it's played with by amateurs.
Here's a video showing how an unvented cylinder explodes
real-free-energy-honest-build-a-solar-batch-heater-