Tesla has agreed to build the world's largest lithium-ion battery storage project in South Australia, billionaire founder Elon Musk said on Friday.

The electric car company, which also makes the Tesla Powerpack storage battery, outbid 91 other companies to win a contract to install a 100 megawatt facility. It will partner with French renewable energy firm Neoen, and get energy from their Hornsdale Wind Farm near Jamestown, South Australia. The facility will be able to power 30,000 homes.

Tesla's Powerpacks will charge using the energy produced from the wind farm and then deliver electricity during peak hours.

Musk announced the partnership at a press conference in Australia, followed by a tweet, in which he said the energy storage facility will be three times bigger than any on the market at the moment.

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"You can essentially charge up the battery packs when you have excess power when the cost of production is very low ... And then discharge it when the cost of power production is high, and this effectively lowers the average cost to the end customer," Musk said at the press conference on Friday.

"It's a fundamental efficiency improvement for the grid."

Musk also said that if the project is not completed in 100 days, it will be free, honoring a pledge he made a few months ago. Lyndon Rive, Musk's cousin and co-founder of Solar City which recently merged with Tesla, said in March that the company could install a 100 to 300 megawatt energy storage project. Rive said at the time that this could be achieved within 100 days.

Mike Cannon-Brookes, the Australian founder of Nasdaq-listed tech firm Atlassian, which is worth over $6.3 billion, then asked Musk on Twitter if he was serious, to which the Tesla boss replied that he was.

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"We actually insisted in the contract that we be held to the hundred days or it's free. That's what we said publicly, that's what we are going to do," Musk said at the press conference.

South Australia has suffered severe storms that has damaged critical infrastructure causing energy blackouts, leaving many residents without electricity. The South Australian government has been seeking to fix the problem with renewable energy.

That is when Musk sent his tweet.

Jay Weatherill, permier of South Australia, a state in the southern central part of the country, hailed the agreement.

"SA (South Australia) has been leading the nation in renewable energy — now we are leading the world in battery storage," Weatherill said in a Facebook post on Friday.

Given the size of the project, Musk said there would be risks involved.

"We thought about this and there is certainly some risk because this is going to be the largest battery installation in the world by a significant margin," Musk said.