So transparency is one of the problems, that's probably the SideBar also one of the cause of battery consumption that is mentioned in many sites. 4% from Aero and, lets say, 2% from the SideBar and you have 6% of Battery Life.
...the Aero theme drives the GPU harder and therefore uses more power. But in the big picture, it's really not that much more. For example, the display on most laptops will consume somewhere between 15-25% of your "power budget" when you are running on battery. Nevertheless, in our testing we've seen that turning on Aero consumes only about 1-4% more of battery life. In terms of making your battery last longer, turning off Aero will not go very far while at the same time costing you some of the cool features that make Windows Vista fun to use, such as Flip 3D, taskbar previews, window transparency and so on.