flywheels don't really work synchronously, though. Or at least, if so, they're not very useful as storage. Inverters can simulate inertia just fine, in fact they can simulate a much larger inertia than the corresponding power of spinning generation, leading to greater grid stability (whether connected to batteries or solar/wind, though without batteries its a little 'brittle' if you run near 100% output, as the power needed may suddenly disappear, so this is probably not good practice for the system as a whole. And you still need some give in your inertia so the grid can actually communicate about supply and demand)