Electrochromic (EC) windows with controllable transmittances according to ambient temperature and solar irradiation strength are highly desired for energyâefficient buildings. However, traditional EC windows operate via consuming electrical energy to trigger the chromogenic reactions, with negligible energy storage ability. Herein, a facile batteryâtype Prussian blue (PB, Fe4III[FeII(CN)6]3)/Zn EC window with excellent EC properties (transmittance modulationâ=â84.9% at 633ânm), remarkable energy storage ability (average output voltageâ=â1.24âV and areal capacityâ=â78.9âmAhâmâ2), fast switching time (tbleachingâ=â4.1âs and tcoloringâ=â4.6âs), as well as an ultralong cycling lifetime (7000 cycles with 60.7% capacity retention and 92.7% transmittance modulation retention) is reported, thanks to the rational electrode and electrolyte design. As an EC window, this device can effectively promote energy efficiency and occupational comfort of buildings by regulating visible light transmittance. As a battery, this device can work as backup power or a component of smart grids, making the buildings more sustainable and functional. The design concept of this bifunctional device is helpful to further the development of nextâgeneration EC windows.