Let's do the math on Earth Hour:
Turn off all lights for one hour. Assume ten light bulbs would normally be on, each 100W. Total savings = 1 kilowatt-hour per household. In our area, this would reduce the next electrical bill by 8 cents due to the annual event.
Here is a more practical approach:
Replace the ten light bulbs with compact florescent bulbs, each consuming 13W. Assuming lights are on 4 hours a day, the annual savings amount to 1,270KWhr, reducing the electrical bill by $101 a year.
The problem with Earth Hour is that it can give participants a false impression. Turn off lights for one hour a year, and they think they are making a difference. They do not. It's the trap of feeling good about doing something immaterial, rather of doing something that makes a difference.
Instead, make it Earth Year, where your home's energy reduction takes place all year around, and is reduced further each year. We replaced our washing machine with one that uses about 11x less energy. LED lighting is replacing CFLs. Work at home.