It's an offer that seems too good to be true.

The world's largest lithium ion battery will be installed in South Australia in 100 days under an agreement between Tesla, Neoen and the State Government.

Tesla will build the battery which will store energy from French renewable company Neoen's Hornsdale Wind Farm.

If the 100-day deadline isn't met, it's free.

The project will be in place before summer.

Is this a legit offer?

Yep.

Earlier this year the billionaire tech entrepreneur said energy storage could solve the state's electricity problems with a Tesla battery farm, and work could be completed within 100 days.

It wasn't long before Australian Atlassian co-founder Mike Cannon-Brookes said on Twitter that he would "make the $ happen" if Musk could guarantee 100 mega-watts in 100 days.

Here's how Mr Musk responded:

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Which prompted this response from Mr Cannon-Brookes:

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It didn't quite happen in seven days, but it did eventually.

No, seriously. Are you really sure?

South Australian Premier Jay Weatherill confirmed it today.

He said it was "confirmed as part of the contractual arrangement".

The 100 day deadline begins once the grid interconnection agreement has been signed.

Making multi-million-dollar business offers on Twitter is all in a day's work for Elon Musk. ( AAP: Ben Macmahon )

What did Musk say?

Here's what he said about size of the project:

"There is certainly some risk, because this will be largest battery installation in the world by a significant margin … the next biggest battery in the world is 30 megawatts."

And here's Mr Musk explaining how the battery works:

"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."

He also confirmed that project would be primarily free if it wasn't delivered in 100 days.

Has he done this before?

He sure has.

In January this year Telsa unveiled its battery storage farm in southern California.

It's got 396 fridge-sized batteries that are capable of powering 15,000 homes, according to The New York Times.

Telsa announced the California project on September 15, 2016 and said it would be delivered in three months.

It was officially opened on January 30, 2017.

Until today's announcement, it was the world's largest battery storage project.

Mr Musk said the South Australian battery project would be "three times further than anyone has gone before".