that's still just inertia though...

Now just using the complicated AC coupling of DC energy, through complex electronics...

All when you could just, big motor with massive mass spinny real fast like, and then when the mass starts spinning the motor, it makes power.

Mechanically, it's probably both cheaper, and more cost effective to just use a flywheel, which is literally going to be an inertial system.

Honestly the easier way to switch from solar DC to grid AC is to just have a flywheel between the grid and the solar power plant. It might not be as efficient as a capacitor bank or super capacitor bank but it's dead simple to implement and it's extremely reliable.