Gas fuel cells usually make a small amount of electricity and (comparably) a large amount of heat, so usually are efficient only in combined heat applications. Of course they can pay-off eventually, but not much sooner than a solar/wind installation. The advantage compared to such systems is that you have reliable power, and no need for batteries (in case heat is constantly used). The disadvantage is that they require fuel, which will probably get more expensive over time. Still it is not a contender for fusion, especially FF, because of the price difference, and carbon emissions.
