Here's an analysis of methane synthesized with water, CO2, and electricity: http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf http://www.sgc.se/ckfinde

Here's an analysis of methane synthesized with water, CO2, and electricity: http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf http://www.sgc.se/ckfinder/userfiles/files/SGC284_eng.pdf They put it at 52% energetic efficiency from electricity. They're starting with a concentrated CO2 stream as a byproduct of a biological waste digester. If they were starting with a more dilute CO2 source like seawater or ordinary air there would also be an energy cost for concentrating the CO2. In my estimation, a reasonable rule of thumb is that it would take 2-4 megajoules of electricity to make one megajoule of common small hydrocarbons (methane, ethane, ethylene, propylene, benzene...) For example, a megawatt hour of electricity is 3600 MJ. If it were used to synthesize the hydrocarbon commodity ethylene at 1/3 efficiency, it would yield about 24 kilograms of ethylene. Bulk ethylene prices in the US in July were $645/tonne, or $15.38 for 24 kilograms. Today a megawatt hour of renewable electricity might cost $30 in a favorable location. So the electricity cost alone of renewable ethylene would be nearly double the all-in cost of petro-ethylene. Probably renewable ethylene would be closer to 4x the cost of petro-ethylene once you add other costs. But, notably, not 100x or even 10x the cost; disposable polyethylene packaging might become too expensive for routine use, absent fossils, but durable goods incorporating polyethylene parts would still be affordable. Higher-value materials synthesized starting from ethylene, like detergents and coolant fluids, would also see a relatively modest price increase. There's a lot of literature on producing liquid fuels and chemicals starting from natural gas and coal. Most schemas proceed through an intermediate called synthesis gas, ("syngas") a mixture of CO/H2 that can be converted into many different products depending on the catalysts used. To get the overall energetic efficiency you'd combine the extant huge literature on synthesis gas conversion with the assumption that synthesis gas is made from the reverse water gas reaction of CO2 with electrolytic hydrogen from water. (Not hard if you're fluent with chemistry and units conversion, otherwise may be challenging. I'd do a table of examples myself if I thought this comment would be more visible.) Here's a presentation that has a schematic view synthesizing renewable hydrocarbons without biomass -- see particularly the slide "Combining Technologies" where it shows that methanol, gasoline, diesel, and methane can all be produced from the same syngas intermediate: http://orbit.dtu.dk/files/51542760/Production_of_Green_Natural_Gas_presentation.pdf http://orbit.dtu.dk/files/51542760/Production_of_Green_Natur...