Mr Musk first made the bold promise in a Twitter exchange earlier this year, as debate raged over South Australia's energy woes.
He now says he'll stand by the pledge, which has been written into the contract to construct the 100 megawatt lithium ion battery.
Mr Musk's company Tesla will partner with French renewable energy group Neoen to build the battery near Jamestown in South Australia's mid-north.
Mr Musk admits the project has its risks, but he's confident he can deliver within 100 days.
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"This a hundred mega watt battery, so the next biggest battery system in the world is 30 megawatts. So we're talking about something that is more than three times as powerful as the next biggest battery station in the world and we actually insisted in writing the contract, that we be held to 100 days or it's free."
