In a move expected to save about 15 tonnes of greenhouse gases every year, the Sydney Opera House is switching off its external floodlights at 2am on weekends, and am on weekdays.

The Opera House briefly extinguished its sail lights for the first Earth Hour in 2007, did it again last year, and will do so once more on the evening of March 28.

But the ghostly vista of the darkened building looming over Bennelong Point will now be a permanent feature of Sydney's pre-dawn skyline.

Dousing the lights every morning will save about 10 megawatt hours of electricity every year, the equivalent of shutting down three small homes.

"I haven't had any complaints from night owls." said the Opera House's chief executive, Richard Evans, who believed other city buildings that still have lights blazing all night should follow.

"It would be difficult to understand why anyone needs to advertise an office block at that time of night, unless you are a nightclub."

Mr Evans said that although the floodlights were now being turned off every morning, security perimeter lights were being left on.

Draping solar panels over the famous sails has obviously never been an option for Opera House staff seeking to curb the building's thirst for resources.

After all, in 2006 research by the global branding specialist FutureBrand found that the building was Australia's most recognisable landmark.

However, its staff have installed new technology that allows it to use water sucked from Circular Quay to absorb heat from its air-conditioning system, saving 15 million litres of drinking water a year.

Water used for cleaning the boardwalk and outside footpaths is now being recycled, and staff have switched to using baking soda and water to clean concrete, cutting the need for toxic chemicals.

In other moves, all 70 shower heads have been replaced with water efficiency models, saving about 600,000 litres a year, and new computers automatically switch to "sleep" mode after 30 minutes.

Printers, photocopiers and fax machines have been replaced with models that print on both sides of paper.