Nearly 40 years after California began offering inducements to people to heat their shower water using the sun, Brad Heavner of the California Solar Energy Industries Association still has to remind them the technology even exists.

When most people think about solar energy, they think about solar photovoltaic panels that make electricity. “But there’s also solar water heating systems that are very effective at using energy from the sun,” Heavner said. Panels that heat water are not sparkle blue. Instead, they are often a dull black inside. It’s all about absorption. And they are larger than panels for electricity.