[manfredo]

Even the most efficient combined cycle gas plants are in the 55-65% efficiency range. The Sabatier process is also at maximum 70% efficient, so end to end efficiency is ~40%.

Furthermore, in order to convert co2 to ch4 you need a source of CO2. Extracting the small percentage of CO2 from the atmosphere is not feasible. You would have to somehow pump the exhaust of the gas plants into a containment vessel, but creating backpressure on a gas turbine interferes with the turbine.

Germany, as well as California, exemplify the problem with intermittent energy sources. Solar and wind are cheap to displace 30-40% of fossil fuels, but going beyond that involves solving extremely difficult engineering challenges.