Switching off lights between 8:30 and 9:30 PM!!!! Will it make any difference? No! if you are the only one to do it. But yes, if you'd be able to convince one more person to do it and of'course you do it yourself too.
Individually it might not affect anything but think when many people like you will do the same. Actually, Earth Hour is to create awareness among the people, so they can take this forward in there daily lives. Then it will really make a difference.
There is no point in discussing global warming and similar issues here. Much has been already said about it. But believe me, it's reality and standing on our faces. Call it ignorance or logic, but even now many (rather most of us) think that this is just an hype created and nothing is happening.
But I feel it is cruel but truth. Just take an example, there was not distant past when we never used to buy water but what's the situation now? We are buying the water not only for drinking but also for our daily usage (Remember, water tanks coming in daily in the morning!!!!).
Early, the better if we realize that we gotta do something about it. Everyone has to come together and pitch in. There is no time left to ignore it anymore for anyone.