I've noticed that since installing Windows 10 on my Surface Pro 2 the battery dies faster than before when running.. However, the really odd thing is its somehow draining the battery when the Surface is OFF. I use the Start Menu -> Power -> Shutdown to turn it off, just like it did in Windows 8.1 but somehow the battery is draining in that condition. After about a day or two the battery will be under 6% when I turn it back on when it was fully charged when I shut down.