Elon Musk's biggest critics argue some of his larger than life ideas may be less than feasible, but on Friday the billionaire put his money where his mouth is.

The tech mogul started on his promise to build the world's largest lithium ion battery plant for South Australia, in less than 100 days — or it's free for the state government.

An agreement between the state of South Australia, Tesla and Neoen, a French wind farm developer, has been signed as of Friday, meaning, Tesla now has 100 working days to install and get the battery systems up and running. The system would store and intermittently release energy to the the region's electrical infrastructure. The storage could provide electricity to more than 30,000 homes.

Previously, Musk had said that if Tesla failed to meet the deadline, it could cost $50 million or more.

CNBC attended the "Powerpack Celebration" held outside of Jamestown, South Australia to commemorate the start of the contract, where it's worth noting that several of the powerpacks had already been installed and event organizers referred to it as an "active site".

"The vast majority of the world is still fossil fuel power," Musk told an audience at the event.

"This is really just the beginning, but I think what this serves as is a good example to the rest of the world of what can be done."

Tesla was selected out of 90 bids from other companies to provide energy storage.

The Tesla 100-day challenge originated when Musk promised on Twitter that he could install the system in a short period of time, after South Australia suffered a state-wide blackout after its renewable energy storage failed.

Earlier this year, Tesla opened a battery storage farm in Southern California, but according to Musk, this project will be three times the size.

The powerpacks were manufactured in Tesla's new Gigafactory in Sparks, Nevada.

Earlier in the day Musk spoke for SpaceX at the International Astronautical Congress in Adelaide, Australia, where he updated his plans send humans to Mars.