Solar water heaters, aside from being very economical, are also generally considered considered to be a far more 'green'. They rely on energy from the sun, and thus can significantly reduce the carbon footprint of somebody's home.
This is a part of the conversation about renewable energy because many people are advocating the construction of these devices, not because it would save money on a family's electric bill, but because it would reduce carbon emissions due to the reduced usage of electricity. Most electricity is dependent on fossil fuels, of which there is a finite amount of in the earth. The sun's energy, on the other hand, will outlast fossil fuels by billions of years.