Electrochromic devices change color and light transmissivity upon application of an electric charge. As such they should prove valuable as "smart" windows that can reduce air-conditioning costs by being darkened to absorb sunlight and reject unwanted solar heat. The biggest drawback is the cost of providing a power source and controls for the windows. NREL researchers have worked on various ways of self-powering electrochromic windows with solar cells or photovoltaic (PV) films (also available for licensing). A recent development is a new form of self-controlled and self-powered photoelectrochromic system that integrates the photovoltaic and electrochromic elements into a single device.