A methane-producing microbial electrolysis cell (MEC) is a technology to convert CO2 into methane, using electricity as an energy source and microorganisms as the catalyst. A methane-producing MEC provides the possibility to increase the fuel yield per hectare of land area, when the CO2 produced in biofuel production processes is converted to additional fuel methane. Besides increasing fuel yield per hectare of land area, this also results in more efficient use of land area, water, and nutrients. In this research, the performance of a methane-producing MEC was studied for 188?days in a flat-plate MEC design. Methane production rate and energy efficiency of the methane-producing MEC were investigated with time to elucidate the main bottlenecks limiting system performance. When using water as the electron donor at the anode during continuous operation, methane production rate was 0.006?m3/m3 per day at a cathode potential of -0.55?V vs. normal hydrogen electrode with a coulombic efficiency of 23.1%. External electrical energy input was 73.5?kWh/m3 methane, resulting in a voltage efficiency of 13.4%. Consequently, overall energy efficiency was 3.1%. The maximum achieved energy efficiency was obtained in a yield test and was 51.3%. Analysis of internal resistance showed that in the short term, cathode and anode losses were dominant, but with time, also pH gradient and transport losses became more important. The results obtained in this study are used to discuss the possible contribution of methane-producing MECs to increase the fuel yield per hectare of land area. Copyright (C) 2011 John Wiley & Sons, Ltd.