Will Earth Hour do damage to power supply system?

The grids can take turning the lights off, by experimental observation: everyday all over the world the lights come up at about the same time for each geographical region, and turn off at about the same time due to the similar sleep schedule of millions. Total black out might overload the system, but the percentage of people who take part in the game is small so even then dangers are minimal.

It is unlikely that the Earth Hour will cause substantial damage to the power supply system. However, it will require a lot of energy and manpower to adjust the power plants to the changes. First of all, generators are automatically taken off the grid if it is detected that the power supply is higher than the current demand. When the Earth Hour ends, power generation companies will have to turn the generators on (which also consumes time and energy) is such a way that they can supply energy to all those who turn their lights on. But they can't do this too rapidly, or the generators will automatically shut down again.

As the vast majority of people aren't stupid enough to fall for the scam/hoax, the effects will be miniscule. And of those who do, as said, their electric lights are only a small percentage of their total power consumption (and most likely they'll compensate by turning to other electrical appliances that consume more electricity than those lights).