>No we don't. Battery systems can stabilize the grid. You need inverters that are designed for this, but it isn't hard to make an inverter that will set then grid frequency and force it correct.

I know in theory we don't, but in practice there is no such real grid anywhere in the world that would run on large percentage of solar. So it seems we do.

I'm talking about real grid as something that powers at least thousands of households. Not a compound with a dozen buildings.

Take Canary Islands for example. They are an ideal environment to setup a 90% solar and battery grid. They have a great climate and size well suited to this. But they still run thermal plants.

Why? Because no one has successfully showed a largish scale frequency regulation/stabilisation with inverters.

See a writeup about the Texas outage a year or two ago to understand how fragile grids with too large percentage of semiconductor sources are compared with rotating masses.