They were cautiously optimistic about SA Premier Jay Weatherill's promise that the battery would "put downward pressure on prices" in a state whose soaring electricity costs have become a national fixation. Tesla's Elon Musk and SA Premier Jay Weatherill announce the world's biggest lithium ion battery. Credit:AAP "Over time, storage can help put downward pressure on prices because it can flatten out peak demand," said Tony Wood, energy program director at the Grattan Institute. "It's a very useful step in the right direction ... but it doesn't solve South Australia's problem, even at that scale." Contractual details, including the total cost of the project and contribution from the South Australian government, were kept under wraps on Friday. The deal was worth "a very large sum of money", Mr Musk said.

But the Tesla chief executive held to his promise - made to much fanfare via Twitter in March - to have the mega-project up and running within 100 days "or it is free". A smaller-scale Tesla battery farm. "That's what we said publicly, that's what we're going to do," he said. "We're talking about something that is more than three times as powerful as the next biggest battery station in the world. "It's a fundamental efficiency improvement to the power grid, and it's really quite necessary and quite obvious considering a renewable energy future." The combination of renewables plus storage is shaping up as a major risk for gas. Credit:Tesla

The battery will harness power from Neoen's Hornsdale Wind Farm near Jamestown, about three hours north of Adelaide, to store 129 megawatt hours of power. Based on a daily household use of 10 kilowatt hours, that would power about 13,000 homes for an entire day, or about 30,000 homes overnight during a crisis. But like the recently announced expansion of the Snowy Hydro scheme, the main purpose of the battery station is to store renewable energy that can later be used to help meet peak demand. "The most worth will come from his battery system in instances when stuff isn't really going right," said Ketan Joshi, an energy data and communications consultant. "Those rare moments when electricity demand is really high and supply is low for whatever reason [and] you need something that can very quickly just turn on and start pushing power out."

Such moments have been frustratingly common in the Festival State, which no longer has coal-fired power stations and is heavily reliant on renewable forms of energy such as wind and solar as well as power from interstate. South Australia was plunged into darkness in September when a freak storm destroyed transmission lines and a key connection to Victoria "tripped" off due to automatic safety procedures. Energy Minister Josh Frydenberg kicked off a political storm by appearing to lay some of the blame on the state's reliance on renewable energy. A final report from the Australian Energy Market Operator concluded reliance on "non-synchronous" forms of energy did, generally speaking, make SA more vulnerable in a crisis, and said the industry needed to "build resilience to extreme events". Praising the "extraordinary collaboration" between Tesla and Neoen on Friday, Mr Weatherill said the battery would "completely transform the way in which renewable energy is stored and also stabilise the South Australian network".

The costs of stabilisation have been a primary contributor to sky-high power bills in the state. Mr Joshi said the mega-battery should "hypothetically" ease pressure on prices "but it will be quite interesting to see what happens". Neoen executive Romain Desrousseaux said the project would demonstrate "large-scale battery storage is both possible and now commercially viable" and that renewable energy can provide "dependable, distributable power". Mr Musk, appearing mindful of critiques about the physical attractiveness of renewable energy projects in Australia, said he would also endeavour to make the battery farm "look good" as a "tourist destination for some period of time". "Australia rocks!" he tweeted later. Tesla's powerpack - what we know

How many houses will it power, and for how long? Tesla says its project will provide enough power for more than 30,000 homes – the same amount that lost power during a state-wide blackout in September 2016. The battery is designed to run at full power for about one hour and twenty minutes. How much will it cost? Costs were not detailed on Friday. Tesla founder Elon Musk has previously quoted US$250 ($AU330) per kilowatt-hour "at the pack level" for 100-megawatt-hour-plus systems. The proposed system would contain 129,000 kilowatt-hours of capacity, meaning the project's cost would start at around $42 million. The head of Tesla's battery division has quoted a cost of about $65 million in the past. Other experts say a system of that size is likely to cost somewhere between $60 and $120 million.

How much will it store? It's the world's largest battery. Tesla says it will store 100 megawatts of charge. The powerpack will be charged using energy generated by the Hornsdale Wind Farm, before pumping the electricity back into the grid during peak hours. Dr Roger Dargaville, Monash University senior lecturer in renewable energy, said the battery would be able to supply about 1/15th of South Australia's peak energy need for one hour and 20 minutes. How long will it take to build? The battery is due for completion in December according to Tesla. Elon Musk famously promised to build the battery within 100 days of signing contracts "or it is free".

How will it affect energy prices? Unclear at this stage, but South Australian Premier Jay Weatherill said it would "put downward pressure on prices here". Dr Dargaville believes the system may in fact increase energy bills if it is privately owned, because whoever owned it would need to find a way to make money on their investment. The battery station would be unlikely to generate significant profit by itself, he said. "It's not designed to save consumers money on their bills – it's designed to help keep the lights on during a 45-degree day in Adelaide." How big will it be?

About 620 of Tesla's Powerpacks will be used to build the battery How long will it take to build? 100 days from the signing of contracts "or it is free", says Elon Musk Loading With Liam Mannix