When solar and wind power input is not meeting demand that seems like a good time to spin up the spare fossil fuel generators, at least as a transitional sort o

When solar and wind power input is not meeting demand that seems like a good time to spin up the spare fossil fuel generators, at least as a transitional sort of thing. Maybe in the near future excess wind / solar power can be stored as chemical fuel, and burned during peak usage. An 80% / 90 % cut in fossil fuel usage or higher would be a huge thing though!