The development and implementation of energy storage solutions is essential for the sustainability of renewable energy penetration in the electrical system. In this regard, power-to-gas technologies are useful for seasonal, high-capacity energy storage. Bioelectrochemical systems for electromethanogenesis (EMG-BES) represent an additional power-to-gas technology to the existing chemical and biological methanation. EMG-BES process can be retrofitted in traditional anaerobic digesters, with advantages in terms of biologic process stability and high-quality biogas production. Nowadays, there are no reported studies of scaled-up EMG-BES plants for energy storage. The present work describes the setup and operation of a medium-scale EMG-BES prototype for power-to-gas, storing energy in the form of biomethane. The prototype was built by stacking 45 EMG-BES cells, accounting for a total volume of 32¿L. It was continuously fed with 10¿L¿day-1 municipal wastewater, and it was long-term operated at different voltage and temperature ranges. A steady-state current density demand of 0.5¿A¿m-2 was achieved at 32¿°C while producing 4.4¿L¿CH4¿m-2¿d-1 and removing 70% of the initial organic matter present in wastewater. Microbial competition between electro-active bacteria and acetoclastic methanogens was observed. Energy storage efficiency was estimated around 42–47%, analyzing surplus CH4 production obtained when applying voltage to the stack. A first order electric model was calculated, based on the results of a series of electrical characterization tests. The model may be used in the future to design the converter for EMG-BES plant connection to the electrical grid. The obtained results show that energy storage based on EMG-BES technology is possible, as well as its future potential, mixing renewable power overproduction, biomethane generation and wastewater treatment under the circular economy umbrella. Peer Reviewed