I recently bought a JOEL Wattforbrugsmåler.

My laptop, a Dell D620, has both Windows and Linux (Dualboot).
I was curious to see whether there was any difference in power consumption during idle, so I decided to put my new device to use!

I removed the battery from the laptop and plugged the charger into my power measuring device before I made sure Windows 7 was running up-to-date drivers, while Linux ran the default drivers.
I then disabled all non-default services and removed any third-party programs from the startup. I booted into each os, let it run for some time (to make sure it was indeed idle) before turning on the clock.

I measured the Watt-Hour usage by each operating system, during the period of one hour.

The power usage, to my surprise, was not the same during the time frame – Linux uses only 71% of the power consumed by Windows, during idle.