I can't see this making much difference to battery life. Maybe if you play games a lot, it might, but in normal use, it still has to light all the non-black pixels to create the image, so if you have something of a size of 1 pixel that needs displaying, the same image at a lower resolution will take use 4 pixels (at the lower resolution), this isn't to say that a lower resolution uses more pixels and is therefore worse for the battery, because it isn't, but the difference for non-gaming use would be minimal at best.