If that's too annoying to store you can get it hot and squeeze it over some nickel to get methane.
Electrolyzation becomes cheaper than mining methane for hydrogen (and thus ammonia) production if solar hits the $0.2-0.3/Watt threshold somewhere (which is predicted to happen in 3-7 years).
It's complicated and expensive, but I'm not sure I'd bet on a sabatier reactor (or hydrogen storage if it gets cheap), an electrolyzer and 4x the solar panels being more expensive than a fusion reactor with the average output of 1 unit of solar.
Plus the sabatier thing means we don't have to upgrade all the heating furnaces and expand the grid to have 8x the capacity.
Fusion will be real handy where power density is king though. And if there's some non thermal way of getting work out of it, I can see it being cheaper.