[ourlordcaffeine]

>And really, you still need a bunch of batteries to handle short-term fluctuations in renewables output.

Not really the case. Renewable output does not fluctuate fast enough to require the fast response of batteries.

Fast responding storage, in the case of the UK pumped hydro, is usually needed due to thermal plants tripping and causing the grid frequency to suddenly nosedive.

[imtringued]

Power plant failure also generally happens due to extreme environmental effects. During those events renewable energies often reach their maximum production capacity and therefore actually contribute to grid stability.