Most of the internet uses flash.
On the mac OS, the latest version of flash uses nearly twice the CPU usage as it does if you were to run windows on the same system. On many laptops today, if you were to install windows xp or ubuntu, you get more battery life as compared to windows 7 or vista or the mac os
GPU acceleration extends battery life as when accelerating flash, the CPU usage is reduced but the GPU usage hovers around 0 and 1% and uses only a tiny bit extra power as compared to just the CPU alone (The GPU is significantly more efficient when it comes to performance per watt)
If you have a windows based laptop, try this, install everest ultimate
using it's power monitor, try this, run prime 95 and watch how much additional power the system uses, now using a GPU or physx based stress test that has very little CPU usage, check the power usage again, you will see that the CPU will use significantly more power.