Superextensivity, where the response of a physical system scales super-linearly with size, originates from collective quantum effects and provides a promising route to augment next-generation quantum technologies. While recent work has demonstrated superextensive behaviour in the coherent dynamics of quantum systems, these effects typically occur on short timescales, prohibiting their practical utility. In contrast, triggering steady-state superextensive effects in, for example, a generated electric current, remains unexplored despite the immediate impact on photovoltaic technologies. Here, we utilise a microcavity quantum battery as an experimental platform that superextensively captures light energy and converts it to an electric current via the incorporation of charge transport layers into the resonant microcavity. This architecture enables, for the first time, a complete quantum battery charge-discharge cycle. We demonstrate that strong light–matter coupling induced by the microcavity leads to superextensive scaling of the steady-state electrical discharging power under low-intensity, incoherent illumination. Our results provide the first experimental demonstration of superextensive light-to-charge conversion in steady-state, highlighting the feasibility of leveraging strong light–matter coupling for enhanced energy harvesting under low-light conditions.