Nick Carr calculates that a Second Life avatar consumes as much electricity as a Brazilian:
If there are on average between 10,000 and 15,000 avatars "living" in Second Life at any point, that means the world has a population of about 12,500. Supporting those 12,500 avatars requires 4,000 servers as well as the 12,500 PCs the avatars' physical alter egos are using. Conservatively, a PC consumes 120 watts and a server consumes 200 watts. Throw in another 50 watts per server for data-center air conditioning. So, on a daily basis, overall Second Life power consumption equals… 60,000 kilowatt-hours….
Which, annualized, gives us [an average avatar consumption of] 1,752 kWh. So an avatar consumes 1,752 kWh per year….. [T]he average citizen of Brazil consumes 1,884 kWh, which, given the fact that my avatar estimate was rough and conservative, means that your average Second Life avatar consumes about as much electricity as your average Brazilian.
Which means, in turn, that avatars aren't quite as intangible as they seem. They don't have bodies, but they do leave footprints.