Nick White, Vista's Product Manager, has responded to an article entitled "Vista, Aero, battery life . . . and Doom" by Dwight Silverman, the tech reviewer at the Houston Chronicle, regarding the impact that the Aero theme has on battery life and performance. White emphasizes that to fully maximize battery life, window transparency is turned off when the machine is put into a power-saving profile but Aero is not because "in the end, doing so is not going to save you much more power".
White agrees with Dwight (yeah, I had to make it rhyme) that the Aero theme makes the GPU work harder and therefore uses more power. However, he states it's "really not that much more" noting the power needed for a notebook's display (15-25% of the battery) to Aero's (1-4%). He believes that saving the extra power is not worth losing "the cool features that make Windows Vista fun to use, such as Flip 3D, taskbar previews, window transparency and so on".
White mentions that battery life varies widely depending on the workload the machine is running. He also underlines that testing the impact of software features on battery life is exceptionally tricky, even by benchmarking standards. He claims this is because the charge a battery requires can vary on a multitude of factors, including the battery technology, how recently the battery was power-cycled, how old the battery is, and even the temperature of the battery when it was charged.