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.