The effects of the environment on the energy storage of supercapacitors as well as the underlying mechanisms have long been neglected. This paper reports that the capacitance of hexagonal-phase tungsten oxide (h-WO3)-based supercapacitors increases by approximate to 17% under solar light. Thorough analyses of the wavelength dependence of the enhancement, capacitive mechanism, energy storage dynamics, and impedance reveal that: i) photoexcited electrons are responsible for the enhancement; ii) the insertion of protons into the large hexagonal tunnels of h-WO3, instead of a surface capacitive process, is greatly facilitated by the photoexcited electrons; iii) the theoretical light-induced capacitance enhancement can reach up to 38% for a h-WO3-based supercapacitor. Moreover, as an application of this finding, a self-powered photodetector based on a h-WO3 supercapacitor is fabricated, wherein the photoexcited electrons serve as the signal for detecting solar light. The device works without an external power source and can be considered as an ultimately integrated power source-sensor system. This work sheds light on the interaction between solar light and a semiconductor-based supercapacitor as well as the concrete mechanisms behind the phenomenon. These efforts also open the door to the design of highly integrated, brand-new power source-sensor systems.