The last few years have seen tremendous growth in solar thermal power plants—huge arrays of mirrors that concentrate the sun's energy onto a liquid which then boils and spins a turbine. The process is generally more efficient than using photovoltaic panels, and new solar thermal plants under construction in Spain and Australia will be among the largest capacity solar plants in the world. Old-fashioned PV panels were starting to look archaic, or at least suitable only for small-scale projects like roof instillations. But not all PV panels are created alike.