One of the big myths about Earth Hour is that it doesn't actually make any difference, that the generators are running anyways. But the generators run on schedules and the operators now take Earth Hour into account. In Ontario, Canada, they dialed back the generators and reduced power generation by more than 6%.
Last year Terry Young of of IESO, the Independent Electricity System Operator, who "balances the supply of and demand for electricity in Ontario and then directs its flow across the province's transmission lines." told me:
Electricity gets produced to according to schedule, and the schedule for Saturday night included a forecast of the expected reduction in demand.
In the City of Toronto, power consumption dipped 15.1% (a lot of people were into it here). In the Philippines, power saved was equivalent of shutting down six coal fired plants for an hour. Even Dubai cut back by 200,000 kWh.
Of course, if people all light candles and drive to Earth Hour parties, that is going to eat up some of those savings. And in parts of the world that don't burn a lot of fossil fuels, it could even be counterproductive. So this year, we are recommending that people cut back on the candles and bike to their parties.