For this Nouveau desktop testing of the power consumption a AMD Sempron box was used running a NVIDIA GeForce 9800GTX. Ubuntu 10.04 LTS was used comparing its Nouveau KMS driver (using the Linux 2.6.32 kernel but with the 2.6.33 DRM back-ported, and no Gallium3D driver in use) to the NVIDIA 195.36.15 binary driver. Ubuntu 10.04 was used for some variation in the test results and the Red Hat Anaconda installer with Fedora 13 Beta would hang with this AMD system when it came to detecting the Serial ATA hard drive.
According to the AC power meter, when the Ubuntu 10.04 desktop was idling using the default Nouveau stack, its power consumption had settled at 133 Watts. With the proprietary NVIDIA driver it was level at 120 Watts.
So with this AMD setup using a NVIDIA GeForce 9800GTX, the Nouveau driver is going through about 11% more power when idling -- granted, this testing isn't as thorough as this morning's power testing due to the Phoronix Test Suite not being able to hook into this AC power meter, but at least we have a better idea now where the Nouveau power consumption stands on the desktop side.