Purpose: To explain that unreliable energy from the sun and the wind is not sustainable as a replacement for conventional power sources (coal, gas and hydro).
Increasing the penetration of unreliable energy from the wind and the sun into the electricity supply can make coal power stations uneconomic and drive them out of business but cannot replace them.
All the states and the Commonwealth government are aiming for net zero emissions from the electricity sector.
Significant progress has apparently been made towards that target with wind and solar power penetration above 20%. On good days the penetration approaches 50% for some time and for some very short periods in South Australia it has been 100%.
We can boast that we lead the world by a wide margin in the rate of increase in our wind and solar installed capacity.
Critical Issues
Statistics on the average penetration or the highest penetration on good days do not mean that ever-increasing penetration is sustainable.
The limiting factor is the near-zero output from wind and solar installations on windless nights.
This is analogous to the lowest point of a flood protection levee.
As long as prolonged periods with effectively zero solar and wind power persist, the march towards net zero is futile. 100% backup from conventional power will still be required, assuming that we want security of supply. That means we will be stuck with a hybrid power system for the foreseeable future until the problem of grid-scale storage is resolved.

The “holy trinity” of transmission lines, batteries and pumped hydro will not keep the system up through periodic shortages of unreliable energy from the sun and the wind.

Transmission lines cannot distribute spare wind power to places that are short because during wind droughts across the whole of SE Australia there is no spare wind power anywhere.
“Big batteries” attached to the grid do not have the capacity to provide grid-scale power for any useful length of time. They provide some stability under the impact of short-term fluctuations in the wind and solar input.
Pumped hydro is too expensive to provide the amount of storage required to support the grid through prolonged wind droughts. It depends on power generated from other sources and it wastes 30 to 40% of the power in pumping and transmission.
The connection of unreliable energy sources to the grid was a mistake in the absence of grid-scale storage.

Coal can be DISPLACED BUT NOT REPLACED by unreliable energy. This is not yet apparent because there is still enough conventional power available to provide 100% of demand most of the time.

The crunch will come when more coal power is displaced.

Additional wind and solar capacity will not help during the windless nights. Governments will have to fund additional gas and/or coal power, or alternatively, we can all adjust to a regime of major load shedding and more rolling blackouts.
Recommendations: That the point and purpose of the RET and subsidies for unreliable energy be reviewed and subjected to thorough public discussion.