Recently, there has been a burst of interest in methane formation from carbon dioxide by bioelectrochemical systems (BES), which has been innovatively utilized for energy generation in a clean and sustainable way. This technology holds a great promise as 4th generation biofuel, a carbon negative source of fuel upgrading feedstock by capture and sequestration of CO2. The 4th generation biofuel does not rely on biomass, therefore, eliminating ever-lasting competition with food production, as land, water and fertilizer are scarce. Here, we design a new bioelectrhochemical system to produce methane from sunlight, waste CO2 and microorganisms. The efficiency of converting sunlight into methane by BES (estimated 70%) combined with a PV cell (15%) will be >10%, which is much higher than that of directly into biomass (only 0.5-3%).