I have two brand new 640's. One with 8.1 and the other with the latest build of 10 from today. Both were completely charged and had the same settings. I played the same one hour video on both of them. The one with 8.1 had 89% battery remaining. The one with 10 had 88% battery remaining. When I have more time I will do a more comprehensive test, but it seems that battery life is very similar based on this little unscientific test.