If you are among the people who face battery draining issue on Surface RT devices running Windows 8.1 preview, there is a temporary workaround to fix it. The solution is strange but according to lots of people, it works!
“Do you have your Bluetooth radio turned off, e.g. through airplane mode? If so, please turn it on as a workaround. Swipe in from right to bring in charms bar, tap on Settings and then on Change PC Settings at the bottom. Subsequent steps to get to the switch to enable Bluetooth: Change PC Settings > PC and Devices > Bluetooth,”
Microsoft is already aware of the bug and should be working on an actual fix. For now, you can use the above method to save power.