This may help:
It’s important to note the phrasing “up to 1,200MW”, meaning the final design and size of the scheme is still in development and the company has not given an indication yet of the planned capacity or discharge duration of the project. Nevertheless, CEP. Energy’s battery storage system would be “up to” four times larger in rated output than the 300MW / 1,200MWh Moss Landing Energy Storage Facility in California, recently inaugurated and currently the world’s biggest battery energy storage system (BESS).
It appears that the Moss Landing has a capacity to supply 300MW for 4 hours so we may be in a position to assume Hunter Valley will be likewise, ergo 4800 MWh