Remember that massive battery Tesla made near Jamestown, South Australia? Well, not only did Elon Musk follow through with the promise to have it up and running in 100 days, and not only is it now the biggest lithium-ion battery energy storage system in the world, but on top of all that it's also already saving the region an absolute shedload of money.

According to global engineering and infrastructure advisory company Aurecon, Neoen's Hornsdale Power Reserve has exceeded all expectations after just one year, having slashed costs of $40 million from the wholesale market.

Aurecon's report, which examined the impact of the Tesla battery, said: "Aurecon's independent report found the system - owned by Neoen and supplied by Tesla - delivered all project objectives, reduced volatility on the state-wide network over the 12-month period and contributed to a reduction of close to $40m in the frequency control ancillary services (FCAS) market."

Early costings estimated the battery cost AUD $65m (US $50m / £37.6m), but news.com.au argues that the construction price at the current exchange rate was likely more to be around AUD $90m (US $65m / £51m).

Either way, it doesn't look like it's going to be long before the battery essentially pays for itself.

Musk was so confident the project would be completed within 100 days, he said if it wasn't ready on time he'd hand it over for free. Credit: PA

Paul Gleeson, Aurecon's energy leader,explained that reviewing the data from Hornsdale Power Reserve's first year of operation has provided huge insights to the capabilities of the new technology.

"[This is] including how these fast response systems can help improve stability, reduce the likelihood of load-shedding events, and contribute to the reduction in wholesale prices," he said.

"The data is telling us that these fast response systems can help us optimise the way Australian's energy system works".

The battery was unveiled last December, and was designed to store renewable energy gathered from wind farms and send it to the power grid when it's needed.

Elon Musk had such confidence in the project being completed on time, that he said if it wasn't ready within the 100-day timeframe he'd promised, he'd hand it over for free.

Billoniare Mike cannon-Brooks had tweeted Musk, asking: "@elonmusk - how serious are you about this bet? If I can make the $ happen (& politics), can you guarantee the 100MW in 100 days?"

Musk's response? "Tesla will get the system installed and working 100 days from contract signature or it is free. That serious enough for you?"