This paper investigates large scale bio-methane generation from renewable sources, mixing hydrogen produced by water electrolysis and syngas obtained by pressurized oxygen blown biomass gasification. Hydrogen is produced by water electrolysis employing time-dependent hydraulic energy related to the water which is not normally used by the plant, named "spilled water electricity". The oxygen, also obtained in the electrolysis process, is employed for biomass gasification to produce syngas: after purification treatments, the syngas is mixed with hydrogen in a chemical reactor to obtain bio-methane. The whole process is optimized here using two different thermo-economic approaches: (i) for the design point analysis of the chemical and thermodynamic significant parameters in electrolysis, gasification, syngas purification and methanation processes; (ii) for an entire one-year time-dependent analysis in order to define the optimal plant size, since the spilled energy and the electrical grid load vary widely throughout the day and the year. The hydrogen generation plant is based on 1 MWe water electrolysers using the "spilled water electricity", when available; in the remaining periods, in order to assure a regular H-2 production, the energy is taken from the electrical grid, but at a higher cost. It is worth noting that the methane produced, named bio-methane, is totally "CO2 free", since it is produced from renewable sources only. Moreover, the optimization method presented here has a general value, thus it can be easily applied to different sizes, economic scenarios and plant locations. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.