Cloud computing hides the environmental impact of computing from the user. When we search using Google our own PC doesn’t suddenly start to cough – the fan doesn’t ramp up, our laptop doesn’t burn through the table. But somewhere in Google processors are using energy to undertake the search. Google is aware of this and tries hard to reduce this cost and its environmental impact.

There is a corollary of this though. When we use peer-to-peer software our processor uses more power and more electricity but we seldom notice. While perhaps tiny in aggregate this can be significant. And unlike Google few of us think about it, or try to use renewable energy to reduce its CO2 emissions.

Let me demonstrate with a quick back-of-the-envelope calculation.

SETI@home (the peer-to-peer application searching for ET) has 5.2 million participants and has produced an aggregate two million years of computing time. Taking an example of power usage for basic computers we can see that the difference between an idle computer and a in-use computer (i.e. where SETI is doing its processing) would be around 20watts ( though perhaps more). Given SETI has run for 17,520,000,000 hours that works out at about 350,400 Megawatt hours or 350.4 Gigawatt hours.

The UK average consumption of Gas and Electricity is about 22,338kWh per household (in 2007).  In the ten years since it started SETI@home has used about as much energy as a town of about 15,000 people would use in Gas and Electricity in an entire year!

Interestingly assuming US consumer energy costs (since most will be in homes in the US) at about 8c per kw/h this is about $28million of electricity! The key points is that this is only about 50c per year per participant – scarcely enough to make them change their SETI screensaver, but highly significant in the aggregate.

And SETI@home has yet to discover anything alien!