Comment by adrian_b

You do not need backup with fossil fuels.

You need backup with hydrocarbon fuels synthesized from water and CO2, like all the living beings have done for billions of years.

Storing energy in hydrocarbons has a lower efficiency for short term storage, but it has a better efficiency for long term storage, in which case batteries would auto discharge.

So energy storage must use a combination of batteries for short term (for a few days at most) together with methods useful for long term (from a few months to many years), including hydrocarbon synthesis, pumped water, etc.

Synthesizing hydrocarbons from concentrated CO2 has already been done at large scale almost a century ago. Now there are much better methods, e.g. using the electrolysis of CO2.

The most difficult part remains capturing the CO2 from normal air and not from exhaust gases where it is concentrated.

This is a difficult engineering problem, but one solved by bacteria billions of years ago, and which probably would already have some good solution if any serious and well-funded research effort would have been done in this direction, instead of only talking about how it would be desirable but without any concrete action.