The World Wildlife Fund (WWF), which spent 10 months organising the event with city authorities and a major newspaper group, said there had been a massive groundswell of support across the city.
Andy Ridley, a WWF Australia spokesman, said the blackout would show Sydney in a new perspective.
He said the event was aimed at raising awareness about climate change and showing that an action as simple as turning out a light could make a difference.
Ridley said: "It's only by joining together that you can make a difference.
"The world has moved into a time where we see climate change as a serious risk, but we want people to realise that it's not all doom and gloom, individuals can take action to help address the problem.
|"One of the things about Earth Hour is really to get the issue out of the scientific and specialist areas and into the mainstream"|
WWF Australia spokesman
"One of the things about Earth Hour is really to get the issue out of the scientific and specialist areas and into the mainstream."
Ridley said WWF hoped to capitalise on the popularity of the event in the longer term with a campaign to reduce Sydney's greenhouse gas emissions by five per cent over the next year.
WWF believes the switch-off could be copied by major cities around the world in a drive to reduce the greenhouse gas emissions blamed for climate change.
Scientists link dangerous global temperature increases to the greenhouse effect, in which gases emitted by burning fossil fuels to produce energy trap heat in the atmosphere.
Organisers say there are a number of simple steps people can take to reduce electricity use, including unplugging appliances such as televisions, microwave ovens and stereos that are normally left on standby.
Other suggestions include switching to fluorescent light bulbs - a move the Australian government committed to earlier this year - and using "green" power sources such as solar.