This article presents a feasibility analysis of a novel operating principle based on fluctuating electricity prices for an existing biogas plant. By investing in an electrolyzer, excess electricity from renewable production can be stored as CH4 by biological methanation of H-2 with CO2 originating from the biogas plant. The main components of the system are an electrolyzer that is connected to an electric grid and an anaerobic digester where the methanation takes place as well as a biogas upgrading unit. First the energy flow of the system was studied, and secondly the operation costs of the system as well as the electrolyzer investment payback time were evaluated. The study showed that up to 40% of the electricity fed into the system can be stored as biomethane, and the system energy flow is most sensitive to the electrolyzer efficiency. The economics of the studied system depend mostly on the electrolyzer investment cost and desired target price for the CH4. The system can be run economically with current electricity prices if the electrolyzer investment costs decrease 60-72% or the price of CH4 increases 20-76% depending on the investment interest and price fluctuation scheme. (C) 2017 Elsevier Ltd. All rights reserved.