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At a Jamestown, South Australia event on Friday, Tesla CEO Elon Musk announced that the company was halfway done installing a 100MW/129MWh utility-grade battery bank near the site of the 100MW Hornsdale Wind Farm.

The battery bank will be the largest grid-tied system in the world when it’s complete. (Currently, the largest grid-tied system is a 30MW/120MWh facility built by AES Energy Storage in Southern California.) The project grew out of a Twitter bet between Australian software billionaire Mike Cannon-Brookes and Telsa and SpaceX CEO Elon Musk. In response to Cannon-Brookes’ incredulity about the speed that Tesla was claiming it could install grid-tied batteries, Musk promised to deliver a system to South Australia, a state that’s suffered debilitating blackouts in recent summers, “in 100 days or its free.”

But “100 days or it’s free” didn’t include time negotiating contracts, and after the bet Tesla went though a competitive bidding process with the state of South Australia for access to an A$ 150 million ($ 115 million) renewable energy fund to cover the cost of the batteries. Earlier this year, Musk gave estimates on Twitter that suggested a 129MWh system would cost $ 32.35 million before taxes and labor. Tesla won the bidding round and partnered with French company Neoen, the owner of the Hornsdale Wind Farm in the mid-north region of South Australia. Musk later commented that if Tesla missed its 100-day deadline, the company could stand to lose “$ 50 million or more.”