An electrochromic material is a material that undergoes reversible changes in its optical absorption properties when small ions are inserted electrochemically into its crystal structure. When thin films made of electrochromic materials are applied in several coatings to glass, for example, they can be used to "intelligently" modulate the transmission of light through the glass, providing solar heat gain when needed or screening it out otherwise. Such smart windows are automatic energy savers, literally darkening when the sun shines, but lightening on cloudy or overcast days. Combining solar cells with an electrochromic film results in a self-powered window, which still has all the intelligence of the smart window.