Why not create a monolithic battery?<br>A 'normal' battery would impractical, but like you were saying about tons of spinning metal, why not create batteries made of giant spinning disks, get a giant metal disk and stick it on a generator, when there's more power you just send it to the generator to spin up the disk, effectively storing the electrical energy as kinetic energy, when there's less power, draw power from the disk, converting the kinetic energy into electrical energy, and since it's as simple as flicking a switch to engage the generator, the delay would be minuscule.