They still rely on interstate power even after getting the big battery, which solved their frequent brownouts but hasn't actually got much capacity: 150MW and 194MWh[0], when the peak grid demand in SA is 2000MW[1] and they've got grid interconnects with Victoria that can deliver nearly 900MW. They make up the capacity with natural gas generators, and their gas usage went up massively after their coal plants went offline[2].
The biggest problem is that SA is pretty flat[3] and doesn't have good sites for dams. They have a grand total of 3MW of hydroelectric capacity, which means they rely on Victoria (and soon NSW) for renewable energy when peak demand exceeds wind/solar generation, as well as their own gas generators and Victoria's coal plants. The big battery can respond faster than the interstate interconnects (which are far away) and the gas generators (which are expensive to run and only turn on when spot prices are high), which lets it fill the same role a big flywheel would in a coal/nuclear plant.
The biggest problem is that SA is pretty flat[3] and doesn't have good sites for dams. They have a grand total of 3MW of hydroelectric capacity, which means they rely on Victoria (and soon NSW) for renewable energy when peak demand exceeds wind/solar generation, as well as their own gas generators and Victoria's coal plants. The big battery can respond faster than the interstate interconnects (which are far away) and the gas generators (which are expensive to run and only turn on when spot prices are high), which lets it fill the same role a big flywheel would in a coal/nuclear plant.
[0] https://hornsdalepowerreserve.com.au/
[1] https://www.aer.gov.au/wholesale-markets/wholesale-statistic...
[2] See https://en.wikipedia.org/wiki/File:Electricity_generation_SA..., which is a year-by-year collection of the data from https://www.energy.gov.au/data/australian-electricity-genera...
[3] https://en-au.topographic-map.com/map-wwnx/South-Australia/?...