The tungsten trioxide monohydrate (WO3 center dot H2O) nanosheets were directly formed on fluorine-doped tin oxide (FTO) substrates without any guidance of seed layer by a novel and quite facile one-step citric acid-assisted hydrothermal method at low temperature (90 degrees C). The WO3 center dot H2O nanosheets possess porous morphologies and good adhesion to the substrates, which would markedly increase the surface area of WO3 center dot H2O and facilitate the ion diffusion during the electrochemical processes. The WO3 center dot H2O nanosheets display superior electrochemical properties of large optical modulation (79.0%), fast switching time (t(c) = 10.1, t(b) = 6.1 s), high areal capacitance (43.30 mF cm(-2)) and excellent cycling stability. Furthermore, bridging electrochromic behavior with energy storage was successfully achieved. Based on the proposed WO3 center dot H2O nanosheets, a smart energy storage electrode was demonstrated, which could monitor the level of stored energy by color changes. The results show great potential of the one-step synthesized WO3 center dot H2O nanosheets for integrated electrochromism and energy storage applications.