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Ideas are easy. Implementations are either hard or expensive.

Converting solar to hydrogen, then to methane, then to electricity requires a pretty big setup. If you want to be efficient, you have to do all that at a large scale with a proper power plant turbine. It will not work well at a scale of one house.

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Is hydrogen -> methane -> turbine more efficient for electricity generation than just hydrogen -> fuel cell?

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Probably not, fuel cells are quite efficient. The problem is, do you want to deal with storing hydrogen? It's very annoying.