I've heard that solar water heating is very efficient, almost all the radiation of the sun on the panel is turned into heat. Also heard Stirling cycle generators (where heat flow from hot to cold is turned into mechanical motion and hence into electricity) are also quite efficient. Though by no means as efficient as solar water heaters. So what happens if you put the two together to produce electricity? You could use the stored hot water in the night too. It must be quite low-cost, certainly the solar panels and water tanks are mass-produced already, the cost is in the Stirling engine. Anyone thought about this?