I recently received a pretty hefty electric bill, and I wasn't sure on what all that electricity (cash) was being spent on. Sure, I speculated, but I wanted to make sure - so I purchased P3 International's Kill-A-Watt Electricity Usage Monitor. It's pretty cool - it will measure Volts, Amps, Watts, Kilowatt Hours over time, among other things.
I immediatly pluged my computers into it, wondering if they were sucking up all of my electricity. After being relieved that it was not the source of my surge in electrical expense (I'm pretty sure it was my air conditioner), I decided to see how various things I did on my computers changed my electricity consumption.
I was surprised. When I compiled a simple C# .NET class library in Visual Studio, my electrical consumption jumped 30 watts while it was compiling. When I did a recursive listing of everthing on my hard drive using Windows PowerShell, it jumped 50 watts of electricity! I'm thinking this is because the hard drive has to do additional work, and thus consumes more energy - but it's really cool to put some relative hard number to it.