But this Saturday it will join thousands of businesses across Sydney in turning off non-essential lights, including the giant neon star on its Pyrmont building, for an hour.

"It's more of a symbol to everyone that we are part of it and agree with the aim of Earth Hour," said Star City's spokesman, Peter Grimshaw.

Of the ASX top 100 companies, 99 will take part in the energy-saving initiative this year, including some of the nation's heaviest polluters.

The only top 100 company that will not sign up is Melbourne-based Tatts Group, which has thousands of poker machines in pubs and clubs around the country. Because the company cannot verify that each machine is turned off, it would be misleading to say it was taking part, a Tatts spokesman told the Herald.

The resources industry has been lobbying the Federal Government in the past year in the hope of further softening its stand on climate change, but many of its biggest corporations are taking part in Earth Hour.

The mining giant Rio Tinto will switch off lights at itsoffices and at some mine sites. The company had an Earth Hour-themed presentation to staff this week, a spokeswoman said.

BHP Billiton will turn out lights in its Melbourne head office, but is leaving decisions over whether to participate to individual mine managers. Woodside Petroleum will switch off its main offices in Perth.

The Minerals Council of Australia willbe included by default when the owner of its Canberra office takes part in Earth Hour.

Some businesses have been inspired by the annual event to go further with their energy savings.

Colonial First State bank has put out a call to its 850,000 investors to take part, and has made some simple but permanent steps towards energy efficiency in its offices, including putting in sensor lights and getting rid of vending machines.

The Children's Hospital at Westmead cannot turn off its lights for safety reasons, but it has reached an agreement with AGL to receive GreenPower renewable energy for an hour.

WWF Australia, which is organising Earth Hour with the support of Fairfax Media, publisher of the Herald, said far more businesses had signed up this year.More on Earth Hour