The NSW Government has called an urgent meeting with an energy company after a second major power outage hit Sydney's CBD in less than a week, with an angry premier declaring ``it's not good enough''.

Power company EnergyAustralia has apologised for the second major disruption to Sydney's power supply this week, which affected about 50,000 homes and businesses.

Properties in the northern part of the city, along with parts of eastern and inner city suburbs, were blacked out at 10.28am on Saturday.

Former Australian of the Year Tim Flannery says if the G20 meeting of world leaders in London does not make a decision on further measures to combat climate change, there will be an even bigger economic crisis.

Mr Flannery is one of the key signatories to an open letter from the Copenhagen Climate Council to the G20, calling on the nations involved to invest in green growth or risk handing huge risks to future generations.

President Obama is facing challenges to American power on multiple fronts as he prepares for his first trip overseas since taking office, with the nation’s economic woes emboldening allies and adversaries alike.