(* assuming that your old, inefficient, Windows desktop computer, and monitor are on 24/7 and you don't have power saving enabled, and you tell a lot of your friends)
"If even 0.1% of all users on Facebook (300,000 users) install Enpower"
This reminds me of a recent article pointing out the fact that there are numbers less than 1 (or 1/10 in this case) [ http://www.techdirt.com/articles/20090928/1101596338.shtml ].
"The Nellis Solar Power System generates 30 million kWH annually"
I have a different perspective for you - 30 million kWH is also about 3 million gallons of gasoline (as burned in internal combustion engines), which is about as much gasoline as this country consumes in 10 minutes. So, not really that much.
EDIT: toned down the vocab, it was too judgemental