This paper investigates hydrogen and methane generation from large hydraulic plant, using an original multilevel thermo-economic optimization approach developed by the authors. Hydrogen is produced by water electrolysis employing time-dependent hydraulic energy related to the water which is not normally used by the plant, known as "spilled water electricity". Both the demand for spilled energy and the electrical grid load vary widely by time of year, therefore a time-dependent hour-by-hour one complete year analysis has been carried out, in order to define the optimal plant size. This time period analysis is necessary to take into account spilled energy and electrical load profiles variability during the year. The hydrogen generation plant is based on 1 MWe water electrolysers fuelled with 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, at higher cost. To perform the production plant size optimization, two hierarchical levels have been considered over a one year time period, in order to minimize capital and variable costs. After the optimization of the hydrogen production plant size, a further analysis is carried out, with a view to converting the produced H-2 into methane in a chemical reactor, starting from H-2 and CO2 which is obtained with CCS plants and/or carried by ships. For this plant, the optimal electrolysers and chemical reactors system size is defined. For both of the two solutions, thermo-economic optimization results are discussed and compared with particular emphasis to energy scenario, economic aspects, system size, capital costs and related investments. It is worth noting that the results reported here for this particular large H-2 plant case represents a general methodology, since it can vary according to their different sizes, primary renewable energy, plant location, and different H-2 utilization. (C) 2013 Elsevier Ltd. All rights reserved.