T he average wholesale price over that window sat above AUD 800 per megawatt hour, with peaks slamming into the market cap of AUD 20,300. A 12 to 16 hour pumped hydro scheme would have carried the state through those peaks for days. Wind supplied barely one percent of the state's power across those four days and batteries drained by evening¹. These events, wind and solar droughts hitting grids moving off coal, are rare. But they are becoming frequent enough to earn a name: power system planners call them dunkelflaute. In German, dunkel means dark and flaute means lull². It describes the moments when the lights go out through lack of power. When renewables go quiet, every grid faces the same test The challenge Australia faces is playing out as coal and gas give way to wind and solar. Iberia provided a live demonstration of the challenge. On 28 April 2025, the Spanish and Portuguese grids collapsed in under 90 seconds, cutting power to more than 50 million people. Investigators pointed not to a shortage of energy, but to a shortage of the services that damp out instability, traditionally supplied by thermal generation and pumped hydro, largely absent from the solar and wind resources running at high penetration⁴.
Britain has moved first on the policy response, ending a four-decade investment drought through a technology- neutral cap-and-floor mechanism designed to underwrite long-duration electricity storage projects.⁵ Different markets, same lesson: replacing coal's megawatt-hours is the easy part. Replacing what keeps the grid standing is the real test. The question is not whether long-duration storage is needed. It is which technologies can keep the lights on when renewables go quiet and do so at the lowest cost over an asset's full life. The problem is not the afternoon. It is the 14 hours after dark According to Australian Energy Market Operator’s (AEMO) 2024 Integrated System Plan, around 90 percent of Australia's coal fleet is expected to retire before 2035⁶. The instinctive fix is to shift cheap midday solar into the evening peak. Batteries do that well. That is the easy part. Across Australia’s National Electricity Market (NEM), demand between 6pm and 8am averages about 308 GWh a night and coal still supplies roughly two-thirds of it, close to 200 GWh. Replacing that overnight block, night after night, including through cloudy winter stretches with little chance to recharge, is a different order of task.
Cheap today, costly for a century
Batteries move the afternoon. Nothing yet moves the night. The evening peak is a shifting problem. The overnight block, repeated night after night through cloudy winter stretches with little chance to recharge, is a different order of task.
Average NEM overnight demand ~308 GWh Coal's share: ~200 GWh
Batteries win on the sticker price, pumped hydro wins once you count the full cost of a lifetime. For roughly 48 hours in June 2026, the wind almost disappeared across South Australia. With little sun to help and no long-duration storage to fall back on, the state leaned on diesel and gas.
6pm
8am
8am
2–4 hour battery 8-hour battery 14-16 hour pumped hydro
Storage duration, hours of continuous discharge
Mike Westerman Global Hydropower Lead, GHD
Source: AEMO Integrated System Plan 2024; GHD analysis.
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