Well, it seems that Ubuntu is more power hungry than Windows 7. Here's a quote from Phoronix's article:

The netbook used for testing was an ASUS Eee PC 1201N with an Atom 330 dual-core CPU, NVIDIA GeForce 9400M ION graphics, 2GB of RAM, a 12.1-inch WXGA display, and a 250GB 5400RPM SATA HDD. The notebook was a Lenovo ThinkPad T61 with an Intel Core 2 Duo T9300, a NVIDIA Quadro NVS 140M, 4GB of system memory, a 15.4-inch display, and a 100GB Hitachi 7200RPM SATA HDD. Ubuntu 10.04 LTS x86_64 and Microsoft Windows 7 Professional x64 were the two operating systems tested on the two mobile devices.

It is unfortunate to see Ubuntu 10.04 LTS going through much more power than Microsoft Windows 7 on these mobile devices, particularly with the modern ION netbook and to still some extent the more mature Intel Penryn notebook with the NVIDIA Quadro graphics. These differences will lead to Windows users noticing a dramatically shorter battery life for those switching to Ubuntu 10.04, which is especially a problem in the netbook era. These were the only 64-bit notebooks we had at the time of testing, so unfortunately we do not know how the ATI or Intel notebook power consumption differs at this point. Fortunately, improving power management has been of interest to more open-source developers in recent times, so hopefully by Ubuntu 10.10 or Ubuntu 11.04 there will be more pleasing news to report to Linux users.

I hope they didn't measure with Compiz running!