Coal was the cheapest source of concentrated carbon monoxide, which is why it was used.

The same technology can be used with carbon monoxide made by reducing the carbon dioxide from air. This requires more energy, but when that is provided by solar energy, this is no longer a problem.

If the energy used to make synfuel is solar, it is an external input and it does not matter much which is the ratio between it and the energy stored in synfuel, except that it determines the profitability of a plant during the first years of operation, as it determines the ratio between the quantity of fuel produced in an interval of time and the installed power of solar panels.

While this ratio determines the time in which the initial investment can be recovered, it matters little for the ongoing expenses required for production, which will vary very little when the ratio varies in a large range, so it has little influence on the production cost after the assets are depreciated.