This study proposes a reliable and self-powered hydrogen (H-2) gas sensor composed of a chemo-mechanically operating nanostructured film and photovoltaic cell. Specifically, the nanostructured film has a configuration in which an asymmetrically coated palladium (Pd) film is coated on a periodic polyurethane acrylate (PUA) nanograting. The asymmetric Pd nanostructures, optimized by a finite element method simulation, swell upon reacting with H-2 and thereby bend the PUA nanograting, changing the amount of transmitted light and the current output of the photovoltaic cell. Since the degree of warping is determined by the concentration of H-2 gas, a wide concentration range of H-2 (0.1-4.0%) can be detected by measuring the self-generated electrical current of the photovoltaic cell without external power. The normalized output current changes are similar to 1.5%, similar to 2.8%, similar to 3.5%, similar to 21.5%, and 25.3% when the concentrations of H-2 gas are 0.1%, 0.5%, 1.0%, 1.6%, 2%, and 4%, respectively. Moreover, because Pd is highly chemically reactive to H-2 and also because there is no electrical current applied through Pd, the proposed sensor can avoid device failure due to the breakage of the Pd sensing material, resulting in high reliability, and can show high selectivity against various gases such as carbon monoxide, hydrogen sulfide, nitrogen dioxide, and water vapor. Finally, using only ambient visible light, the sensor was modularized to produce an alarm in the presence of H-2 gas, verifying a potential always-on H-2 gas monitoring application.