[fastball]

Do you mean the methane-powered Starship, for which they plan to use the Sabatier process to get CH4 from the atmosphere?

[iknowstuff]

Is the process expensive compared to just buying CH4 otherwise?

When Tesla announced superchargers, they were all about solar panels and cars powered by sunlight, but for the serious rollout they had to go with the more easily scalable, cheaper option, so very few superchargers have solar panels after all.