To my shame I completely missed earth hour this year -  http://earthhour.wwf.org.uk/about_earth_hour/aboutearthhour/

I must try to do better next year. I must remember to turn the lights back on again too, after the "lights off hour" is over. When I spent earth hour with a group of friends, we all so much enjoyed chatting to each other by candle light that it was way past the hour before we switched our lights back on.

I somehow imagine that everyone turning the lights on again is the bit that registers most with the power providers. I imagine it could even attract the notice of other users if the "turning back on"  was dramatic enough to suddenly put a heavy demand on supply and make all the lights dip for a moment.