As far as the power consumption goes when idling within each of the desktops from the Intel Core i7 notebook with Quadro graphics backed by the NVIDIA binary blob, the battery power consumption was effectively the same.
The first graphics test was OpenArena. The fastest NVIDIA graphics performance was with the LXDE desktop followed just behind was Openbox and Xfce. The Unity and Unity 2D desktops were a ways behind, but coming in a distant last place was KDE 4.8.2. Again, this is with each of the desktop's default settings and default window manager settings. KWin does allow full screen windows to be un-redirected and other options that can potentially boost your graphics performance.
During the OpenArena run, the Unity and Unity 2D desktops were both going through about 2 Watts (4~5%) more power than the other desktop environments that were tested.
When running Nexuiz, the Unity desktops and KDE remained a ways behind the lightweight Xfce, LXDE, and Openbox desktops on Ubuntu 12.04.
The faster and lighter desktops also provided a slight power-savings.