Tony Walsh has, as others do, some doubts about whether Second Life is sustainable as a business. But he also poses another question that I hadn’t come across before: “Is Second Life sustainable ecologically?”

He quotes Philip Rosedale, the head of Linden Lab, the company behind the virtual world: “We’re running at full power all the time, so we consume an enormous amount of electrical power in co-location facilities [where they house their 4,000 server computers] … We’re running out of power for the square feet of rack space that we’ve got machines in. We can’t for example use [blade] servers right now because they would simply require more electricity than you could get for the floor space they occupy.”

Walsh notes that on average there are between 10,000 and 15,000 avatars in Second Life at any given time, a number that’s growing rapidly. He wonders: “How much power do 15,000 human beings consume daily compared to 15,000 avatars?” Hmm. That’s an interesting question.

So let’s do the math.

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:

(4,000 x 250 x 24) + (12,500 x 120 x 24) = 60,000,000 watt-hours or 60,000 kilowatt-hours

Per capita, that’s:

60,000 / 12,500 = 4.8 kWh

Which, annualized, gives us 1,752 kWh. So an avatar consumes 1,752 kWh per year. By comparison, the average human, on a worldwide basis, consumes 2,436 kWh per year. So there you have it: an avatar consumes a bit less energy than a real person, though they’re in the same ballpark.

Now, if we limit the comparison to developed countries, where per-capita energy consumption is 7,702 kWh a year, the avatars appear considerably less energy hungry than the humans. But if we look at developing countries, where per-capita consumption is 1,015 kWh, we find that avatars burn through considerably more electricity than people do.

More narrowly still, the 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.

UPDATE: In a comment on this post, Sun’s Dave Douglas takes the calculations another step, translating electricity consumption into CO2 emissions. (Carbon dioxide, he notes, “is the most prevalent greenhouse gas from the production of electricity.”) He writes: “looking at CO2 production, 1,752 kWH/year per avatar is about 1.17 tons of CO2. That’s the equivalent of driving an SUV around 2,300 miles (or a Prius around 4,000).”