It's not clear (yet) what a 100% clean energy powered world would use to cover the last couple of percent of demand when loads peak and/or variable generation troughs for extended periods.
It'll be some combination of demand management (which isn't nearly as horrifying as people make it out to be), pumped hydro, long-duration batteries like iron-air, but also possibly burning hydrogen or hydrogen-derived synthetic fuels (produced by electrolysis when hydrogen is abundant) and/or biofuels in turbines.
There is a time- honored, straightforward way to deal with the last two percent problem, which is to overbuild by a couple of percent or so.
Somebody calculated that a home in UK needs 1 Megawatt-Hour battery to backup solar energy during the winter. I suspect in 10 years that may cost below 25K, a small fraction of the property cost.