The companies ALSO charge a daily access fee of around $1.20, which is where the vast majority of the cost associated with maintaining the grid would be accounted for.

I don't know offhand sorry, but I imagine there's a LOT of energy lost in those systems due to the physics of mechanical energy transfer. That being said, I've heard there's some very efficient underground flywheel systems that have been implemented in the last few years.

The problem with this is that the energy companies are doing everything they can to prevent solar being an attractive option. Most companies offer 4-6c/kWh for energy exported to the grid, while charging you 25-30c/kWh for energy imported from the grid.

So the best thing you can actually do with your energy is fully use it, but due to the constant sun in AU, that's nigh impossible - even with a small 5kW system.

If energy was brought back to a publicly owned service, I imagine they would very quickly build large battery banks to absorb the extra, and distribute it at night when it's not being generated as much. There are some companies that are working towards this system (known as a Virtual Power Plant - or VPP), but they are always heavily in favour of the company implementing them, not the customers.