It took 100,000 construction workers a quarter of a century to bore through the Snowy Mountains to build Australia’s largest hydroelectric scheme. The vast nation-building project links nine power stations and 16 dams via a network of 145km of tunnels and pipelines, providing irrigation water and energy that has helped transform the country’s economy since it began operating in 1974.  Now, almost half a century later, Australia’s newly elected government is placing the state-owned Snowy Hydro plant at the vanguard of another energy transition by transforming it into a massive “water battery” that will help keep the lights on as the country shifts from an electricity grid based mainly on fossil fuels to one built around renewable energy.

“We are betting the whole company on it,” says Paul Broad, Snowy Hydro’s chief executive, who confounded critics by persuading Canberra to back an expansion worth more than A$5bn ($3.5bn) that was dismissed just a decade ago as too expensive and risky. “You can’t have renewables without reliable storage and the best form of storage is water.”

Pumped hydro is a century-old technology, which provides about 95 percent of worldwide energy storage linked to electricity grid systems. It works by using excess or cheap power at off-peak times to pump water into raised water basins, from where it can be released to generate electricity·when demand and prices are highest. The need for storage is expected to accelerate massively with the greater use of renewables –                                 and while there has been a lot of hype surrounding lithium batteries, pumped hydro is expected to remain the backbone of the renewables revolution.