A quick search on TreeHugger shows that 'solar power' most often occurs in connection with producing electricity and only secondarily in connection with solar water heaters. That's unfortunate, according to scientists at the Dr Panjabrao Deshmukh Agricultural University in India, because using the sun's power to heat water is a far more efficient process and offers a much quicker return on investment:
According to their analysis, published in the International Journal of Global Energy, as the payback time for a solar water heating system is about two years (with a lifespan of 20), this is a far more cost-effective use of solar energy in the developing world than using it to generate electricity. Compared to electrically heated water, solar water heating is 57% of the internal rate of return, the scientists found:
Solar energy is the only renewable energy source that has wide range of uses with commercial viability. The use of solar energy for thermal purposes is the most cost-effective way of utilizing the resource.