Big, four-hour battery nears completion on world’s biggest isolated grid

A new 100 megawatt (MW) and up to four-hour big battery could be ready to start storing and dispatching renewable energy on Australia’s most isolated grid in time for summer, with construction of the project said to be nearing completion.

Located just south-west of the town of Merredin in the WA Wheatbelt region, the 400 megawatt-hour (MWh) Merredin battery energy storage system (BESS) began construction in August 2025, after the Atmos Renewables project reached financial close July.

WA energy minister Amber Jade Sanderson said on Facebook on Sunday that the BESS is almost ready to be switched on, with construction wrapping up and the battery set to be energised in September before final testing.

“Construction is wrapping up, with the battery set to be energised in September before final testing ahead of summer,” the minister said in a FB post.

“Once fully operational, it will help bring more renewable energy onto WA’s grid and support a stronger, more reliable power system when demand is at its highest.

“More storage. More renewables. A stronger energy system for WA.”

Strategically located alongside the 650 km Muja to Kalgoorlie 220-kilovolt transmission line, once operational the battery will provide critical support to the local network and help enhance the security of supply in the state’s main grid, the South West Interconnected System (SWIS).

Western Australia, one of the largest isolated grids in the world, is quickly ramping up its battery storage capacity.

Already, WA is home to one of Australia’s largest and longest duration battery projects – the 560 MW/2,240 MWh Collie battery, which can charge and discharge the equivalent of 20 per cent of the average demand of the SWIS.

The state is also home to a number of smaller batteries, including two stages of the 100 MW/200 MWh and 200 MW/800 MWh Kwinana batteries, a separate 500 MW/2,000 MWh battery in Collie operated by Synergy, Altina’s 100 MW/200 MWh Wagerup battery, and the 50 MW/200 MWh Cunderdin solar battery.

WA’s Regional Development Assessment Panel gave the green light to a massive six-hour 800 MW/4,800 MWh BESS set to be built in Red Gully earlier this year, while Melbourne-based developer Mint Renewables has also submitted a development application for an eight-hour 100 MW/800 MWh system in the Shire of Kojonup in WA’s Great Southern region.

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