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On Friday, Tesla switched on the massive 100MW, 129MWh battery installation it built in South Australia, just as the state is about to head into the grid-taxing summer season. The installation was completed last week, ending a bet between Tesla CEO Elon Musk and Australian billionaire Mike Cannon-Brookes. The bet was made in March, when Musk tweeted to an incredulous Cannon-Brookes that Tesla could build and install a massive lithium-ion battery installation in "100 days or it is free."

Last year in South Australia was a bad one for blackouts caused by weather and grid failure. Some blamed the intermittency of renewables for the issues—wind has become a big player in the region's energy mix. Storage is a way to smooth out that intermittency. Tesla's installation is situated next to the new Hornsdale Wind Farm, a swath of land that can provide up to 100MW of power at full capacity.

Although the bet was made in March, it wasn't a given that South Australia would allow Tesla to build its battery installation. The electric vehicle and battery company went through a competitive bidding process to have access to an A$150 million ($115 million) renewable energy fund for a 100MW storage contract. Tesla meant to be competitive, though. In March, Musk tweeted to Cannon-Brookes that an installation would cost “$250/kWh at the pack level for 100MWh+ systems,” bringing a 129MWh system to about $32.35 million before taxes and labor. Still, South Australia has not revealed what it's paying to Tesla in total.

The 100 days didn't start after Tesla won the contract in June, though, because the company didn't count time negotiating contracts. The company announced that its 100 days started in October, after the Australian Energy Market Operator (AEMO) approved the project. At that point, Musk said that Tesla was already about halfway done installing the batteries at the site.

It was no surprise that Musk won his bet last week. But even with all the lead-time, the installation was very quickly installed, especially given that it's the largest grid-tied lithium-ion battery installation in the world. Experience may have helped: in January of this year, Tesla completed an 80MWh battery installation in Southern California in just 130 days.