Delhi reports that : http://timesofindia.indiatimes.com/NEWS/City/Delhi/Delhi-to-observe-Earth-Hour-every-month/articleshow/4350616.cms
1 Earth Hour = 60 minutes of power = 10,000 MW saved.

but why just an hour ??
Instead, why not have an earth minute every day of the year ?

so
1 minute x 365 days = 365 minutes = 6 earth hours = 6 x 10,000 MW = 60,000 MW saved.

so
5 minutes x 365 days = 365 minutes = 30 earth hours= 300,000 MW saved

and
10 minutes x 365days = 3650 minutes = 60 earth hours = A whopping 600,000 MW saved !!

.
.
.
It's not that complicated is it ? What say Delhi ?