Windows let in light and with the light solar heat. There are many forms of passive design control of windows to maximize the light, collect the heat, and maximize or minimize the heat that enters through the window. Heat is energy and can be theoretically transformed into power and electricity. A new type of transparent solar film developed by the U.S. Department of Energy could turn windows into clean electricity generators. Harnessing the power of the sun means placing solar collection devices where they are most likely to be in direct contact with its rays. For many years, that ideal place has been the roofs of homes and businesses, but newer technologies are aimed at expanding this range to windows as well.