Across capital cities nationally, an average of 58 per cent of adults participated in the event, according to a survey by AMR Interactive.

But in Canberra the participation rate was 73 per cent, with the next closest city being Sydney at 59 per cent, followed by Melbourne at 56 per cent.

The high number of Canberrans switching off their lights resulted in an 11.4 per cent drop in energy consumption.

They joined millions of Australians and others worldwide in turning off their lights between 8pm and 9pm in a bid to fight climate change as part of Earth Hour, the largest voluntary power reduction event in history.

According to ActewAGL chief executive John Mackay, in just an hour Canberra reduced its greenhouse gas emissions by 36 tonnes the equivalent of taking 8.4 cars off the road for a year.

To congratulate the city, the company will buy enough green energy to offset the remaining essential energy used during Earth Hour, equivalent to taking 65 more cars off the road for a year.

"I am confident the Earth Hour message will live beyond the event itself, with long-term commitments being made by individuals, businesses and governments to save energy into the future," Mr Mackay said.

Before Earth Hour in Canberra, more than 2500 individuals and 330 businesses, organisations and schools from the ACT registered to participate.

All significant institutions in the capital switched off their lights, including Parliament House, Old Parliament House, the National Library, the National Gallery, the High Court, the National Museum and the Royal Australian Mint.