Earth Hour 2011

Measurement of reduction in electricity use
According to figures from EnergyAustralia, a local utility, mains electricity consumption for the 2007 event in Sydney was 10.2% lower during the Hour than would be expected given the time, weather conditions and past four years' consumption patterns. The Herald Sun equated this with "taking 48,613 cars off the road for 1 hour." Critics, most notably Columnist Andrew Bolt, labelled this as "A cut so tiny is trivial - equal to taking six cars off the road for a year". In context, the six cars equates to there being six fewer cars on the road at any given point of time in the day or night. In response to this criticism, the organisers of Earth Hour counter that "If the greenhouse reduction achieved in the Sydney CBD during Earth Hour was sustained for a year, it would be equivalent to taking 48,616 cars off the road for a year." and they also note that the main goal of Earth Hour is to create awareness around climate change issues and "to express that individual action on a mass scale can help change our planet for the better." and not about the specific energy reductions made during the hour being all that's required.
The 10.2% figure was itself challenged by David Solomon, a finance student at the University of Chicago. Without citing data sources or the analytical methods he claims to have used, Solomon says he used eight years of electricity usage data to conclude that the Earth Hour-inspired drop was 6.33%, and that after other potential factors were taken into account, 2.10%, "statistically indistinguishable from zero." In some areas in the Northern Hemisphere, it will be twilight at 8 p.m., removing some of the advantages of the event.