"Let's first consider what a family is saving by turning their 'nonessential' lights off. To do this we'll do some very simple math: being generous, one 100-watt light bulb costs around a penny per hour to operate and generates around 1 tenth of a pound of carbon. If one therefore ran 10 light bulbs, then, a little over 1 pound of carbon would not be emitted and a dime would be saved by Earth Hour.
The question all individuals should ask themselves is, since this decision doesn't occur in a vacuum, what alternatives are present to sitting in a dark, lightless house. What is this family going to do in the dark? They might burn candles — and if they're paraffin, they are based in fossil fuels and will provide a dubious savings in either carbon or monetary cost. If they were to use flashlights instead of light bulbs the result would be an increase in carbon because candles and light bulbs are less energy efficient.
What if that family drove for 15 minutes, went and watched the stars, and drove back home? That trip would cost whatever a half-hour's worth of driving costs. If this trip took a gallon of gas, that would create around 20 pounds of carbon dioxide and would cost over $2 … for a dramatically increased cost in cash and carbon.
One can wonder further about the ramifications on safety and efficiency of turning off "nonessential" lights and conclude that no meaningful savings can be had. Because households are responsible for only 25% of the total electric lighting, we must also consider the commercial and industrial sectors. Instead of being at home, in darkness, will shopkeepers have to return to their stores to guard them for an hour? Will adults turn on lights in dark rooms when they need to move around the house? Will candles generate more fires? Obviously none of these unforeseen circumstances — all of which seem plausible — will improve our carbon footprint."
Now if they suggested that everyone went to bed an hour earlier and assumed that they would still get up after it was light and therefore save an hour of electric light then it could reduce the carbon output but possibly not significantly because residential use is only a small portion of total use.