It doesn't make the slightest bit of a difference this, it just means that power being generated at the plants isn't used that is all.

Earth Hour is a waste of time and energy : Mile High Soapbox

"Power companies plan their power generation based on historical averages and will have that power available to deliver during Earth Hour whether it is used or not. They will also have reserves available in case demand is greater than the historical averages.
So turning off your power and non-essential appliances for an hour will not save any electricity or reduce carbon emissions for that one hour. Nice idea, just wrong."