The last stage of South Australia's Hornsdale wind farm comes online today. With that, the ACT, more than 1,000 kilometres away, will "officially" be powered by 100 per cent renewable electricity.

Key points: About four-fifths of the power on the eastern-seaboard grid that Canberrans use is from coal or gas

About four-fifths of the power on the eastern-seaboard grid that Canberrans use is from coal or gas But the ACT pays for enough renewable electricity generation to account for the city's entire use

But the ACT pays for enough renewable electricity generation to account for the city's entire use A fossil-fuel-free grid is possible in the future with battery storage and smarter use of power

But that does not mean Canberra is some sort of commune running off the grid.

Rather, for every watt of power the ACT consumes, it pays one back through its renewable investments around the country.

Climate Council senior researcher Tim Baxter said ACT homes still consumed energy produced by coal and gas plants.

"What it actually means is that the ACT has taken a choice to incentivise renewables in the grid that are capable of providing enough capacity to match its entire demand — they're not suddenly becoming an island," he said.

The tricky equation to get to 100 per cent

The ACT now pays for enough renewable energy generation to account for its entire electricity use. ( ABC News: Jake Evans )

Only about five per cent of the territory's electricity is generated within its borders, by a few solar farms and rooftop panels on Canberra homes.

The rest comes from the national electricity market — the grid that powers the eastern seaboard — and four-fifths of the grid's power comes from non-renewable sources.

The ACT funds five different wind farms around Australia to feed energy into the national grid to make up for what Canberra consumes.

Those investments, together with Canberra's own solar, put a stop to about 40 per cent of the ACT's carbon emissions, compared to what it would emit if it just bought traditional power from the grid.

The ACT-contracted solar and wind farms cover 77 per cent of the capital's electricity consumption.

The shortfall is made up by the ACT Government's mandatory contribution to the national renewable energy target, where it buys the equivalent amount of renewable power for 21 per cent of its total consumption.

Climate Change Minister Shane Rattenbury said that, from today, Canberrans could trust that all their power was paid for in renewable energy.

"When you use power in the ACT, you can know that it's coming from clean renewable sources — that said, there is still real value in being energy efficient," he said.

Canberrans can't leave the lights on — yet

As the vast majority of the electricity produced on the grid is from non-renewable sources, the power that Canberrans consume at any time may well have been generated by burning coal or gas.

ACT Climate Change Minister Shane Rattenbury says the next step is effective storage. ( ABC News: Jake Evans )

But, this has the advantage of allowing the ACT to lean on the grid when it ramps up production in times of high demand, something it could not do if it sourced its power directly from wind and solar farms.

"There will be times where it won't be windy in certain areas or it won't be sunny, and that is the nature of renewable energy," Mr Rattenbury said.

"But if we build a smart system with the right storage and the right ability to move power between the jurisdictions, we can create a grid that is 100 per cent renewable."

It means that, like a carbon offset paid by a passenger catching a plane, the ACT is paying its way, but it is not free from fossil fuels yet.

And because of that, switching on the heaters after Anzac Day still contributes to overall demand on the grid, and therefore the total amount of power that must be produced.

But Mr Baxter said there was no reason renewable sources could not power the entire grid in the future.

"There are things that need to be overcome … [there are] transmission constraints in various places, where the transmission lines aren't capable of carrying enough renewables at the moment — but that's just an upgrade, it's not a barrier," he said.

An electricity grid free from any reliance on fossil fuels "is technically possible". ( ABC Central West: Gavin Coote )

Mr Rattenbury said achieving that required backing from other states on the grid.

"With the right storage and the right infrastructure behind it, you could do it, it is technically possible," he said.

"But our system is not built like that at the moment, and so that is the transition we need to make."