Phoronix compares the power consumption of Ubuntu 8.04 "Hardy Heron" and Windows Vista:
In Q4'07 we had looked at Ubuntu's power consumption with all
of their Linux releases going back to Ubuntu 5.04. While Linux has
improved in recent years when it comes to power efficiency and
optimizations, more processes running on the desktop had canceled out
any real power improvements. Following that article was a look at power
consumption between Windows and Linux. We had used an old desktop system
in that comparison and Ubuntu 7.10 was consuming the most power while
idling but Fedora 8 Test 3 had consumed the least amount of power and
had beat out both Windows XP and Vista. While using the desktop,
however, both versions of Windows had consumed less power than Fedora
and Ubuntu. With Ubuntu 8.04 LTS now available, we have decided to run
another simple power comparison. This time we are using a Lenovo
ThinkPad notebook and an AMD server as we see whether Ubuntu Hardy Heron
or Microsoft Windows Vista consumes less power.
Read more over here. The site tested the power consumption of both operating systems in idle and found out that Ubuntu uses 5W less on the notebook than Windows Vista and uses 2W more on the AMD server.