The "world's biggest" lithium ion battery is to be built in South Australia by Tesla and French company Neoen.

It is to be close to the French renewable energy company's wind farm near Jamestown and ready by the start of summer.

What is it?

An array of lithium ion batteries will be connected to the Hornsdale wind farm, which is currently under construction in SA. It will look like a field of boxes, each housing Tesla commercial-scale Powerpack batteries.

This is what Tesla envisions South Australia's biggest lithium ion battery and wind farm will look like. ( Supplied: Tesla )

The array will be capable of an output of 100 megawatts (MW) of power at a time and the huge battery will be able to store 129 megawatt hours (MWh) of energy so, if used at full capacity, it would be able to provide its maximum output for more than an hour.

It will be a modular network, with each Powerpack about the size of a large fridge at 2.1 metres tall, 1.3m long and 0.8m wide. They weigh in at 1,200 kilograms each.

How will it stack up against the next biggest?

Sorry, this video has expired Elon Musk says the battery system will be "three times more powerful" than any other on Earth.

It will have just slightly more storage than the next biggest lithium battery, built by AES this year in southern California.

The battery storage facility built by San Diego Gas and Electric and AES. ( Supplied: San Diego Gas & Electric )

But Tesla's 100 MW output would be more than three times larger than the AES battery and five times larger than anything Tesla has built previously.

The largest lithium ion battery storage system that Tesla has built to date sits on a 0.6-hectare site at Mira Loma in southern California.

American electricity company Southern California Edison was also involved. It has a storage capacity of 20 MW, or 80 MWh, and is said to be capable of powering 15,000 homes.

The California array took three months to build.

Tesla's lithium ion battery storage project in Mira Loma. ( Supplied: Tesla )

Tesla says the lithium ion batteries in the Jamestown array will have a life of about 15 years, depending on their usage and how aggressively they are recharged.

The company says the battery components are replaceable and the circuitry should last 20 to 30 years.

Why is it being built in SA?

The state has faced widespread load shedding at times and a statewide blackout last September, prompting the SA Government to prepare a $550 million energy plan for the future.

SA wanted expressions of interest from the private sector to build and operate a big battery. Taxpayers would contribute and the Government would have the right to take energy from the battery when demands dictated.

The state got 91 expressions of interest from around the world. Neoen and Tesla's bid was assessed as the best.

The Tesla Powerpack system is modular. ( Supplied: Tesla )

Before the tender process opened, Tesla boss Elon Musk made an intriguing pitch, via Twitter, that his company could deliver the battery within 100 days — or it would be delivered for free.

How will it be used?

Neoen said the battery would primarily provide stability for the power grid, something traditionally the domain of coal, gas and hydro, rather than wind or solar.

Since the closure of Port Augusta's coal-fired power stations last year, the market has tightened in SA and prices have been very high.

The big battery should be able to provide competition in this market to help drive down prices, with an eventual flow-on to power consumers.

In return for its undisclosed financial contribution to the battery project, the SA Government would gain the right to tap up to 70 per cent of the battery's output at times.

That energy could be used to avoid future load-shedding blackouts in summer if electricity demand is forecast to outstrip supply.

What are the risks?

Reputation is at stake for both Tesla and the SA Government. Elon Musk has staked his credibility on building a big, working battery within 100 days. Tesla has met such a challenge before, albeit on a much smaller scale.

The Government's credibility hinges on fixing the ongoing power woes that have plagued SA.

If the battery is not operating by the start of summer, South Australia might be vulnerable to blackouts in the lead-up to Jay Weatherill's Labor Government heading to the polls next March.