AnandTech has done an apples to apples comparison of the battery life of AMD and Intel based notebooks, you can check it out over here. For the test the website compared the AMD-based Gateway NV52 and the Intel-based Gateway NV58, which feature nearly identical specifications. The Intel-based system costs $80 more, but testing reveals it offers between 20 and 35 percent more battery life than the AMD-based system.
Considering these systems are as close as we can get to "identical", AMD takes a real pounding in battery life testing. The closest result (the idle test) has the Intel platform providing 20% more battery life, while the best Intel results (DVD playback and heavy web surfing) give about 35% more battery life. Averaging all five results, the Intel-based NV58 delivered 28% more battery life than the AMD-based NV52. Ouch.