I have a Asus laptop with this specs:
CPU: AMD A6-3420M (4x1,50 a 2,40 GHz / 4 MB caché).
Resolution: 15,6" / 1366x768.
RAM / HDD: 4 GB RAM DDR3 / 500 GB (5400 RPM).
graphics: AMD Radeon HD 6520G + 7670M de 1 GB DDR3.
free driver with the battery lasts 50% less than with the proprietary driver, why is that?
Because the open drivers only recently got good enough for almost everything and the power management is one area where they are still lacking.
Graphic cards are complex and although some documentation exists it still needs someone dedicated to do it. There are some hopes the amd guys release some more code that gives a better implementation but that code is still passing legal reviewing ( go check phoronix.com for more information ).
If you really care for the battey stay with the proprietary drivers if not go with the open ones as they work for everything else (still lacking for more demanding 3D, like some games).
Owner of Asus 1215B :: User of Xfce :: Vasco Dias @blog