A solar farm that has been billed as Australia's largest has officially opened at Royalla, south of Canberra.

The 20 megawatt Royalla Solar Farm was developed by a Spanish company, is made up of 83,000 solar panels and has the capacity to power more than 4,500 ACT homes.

The ACT Government said it was the first large-scale solar farm to be connected to the national electricity grid.

Environment Minister Simon Corbell said it was an important step towards achieving the ACT Government's target of 90 per cent renewable energy for the territory by 2020.

"Over its lifetime the farm will save over half a million tonnes of greenhouse gas emissions," he said.

"This is a nation-leading project. This is the largest operation solar farm in Australia to date.

"It is expected to generate an average 37,000 megawatt hours of renewable energy each year for the next 20 years."

The project is owned by Spanish company Fotowatio Renewable Ventures (FRV) and the opening was attended by the Spanish foreign minister Jose Manuel Garcia-Margallo.

"If we want to have sustainable development, we must take care of the environment," he said.

The Royalla solar farm is made up of 83,000 photovoltaic panels. ( ABC News: Jonathon Gul )

"I'm absolutely convinced that it's very important to go down this path."

Andrea Fontana from FRV said large scale solar farms relied heavily on government support.

"It's absolutely important to have support and long-term stability to make investment decisions," he said.

"It's impossible to attract capital and investors' confidence without long-term policies."

The ACT's moves towards greater reliance on renewable energy have come at a cost for Canberrans.

The ACT Government said the cost of reaching its renewable energy target of 90 per cent by 2020 was expected to peak at about $4 per household per week in 2020 before declining.

It said that cost would be offset by weekly savings of about $4 per household through energy efficiency programs.