There have been reports of the battery on Surface RT draining very quickly on some machines running the Windows 8.1 Preview while the device is in Connected Standby.
Well a post in the Microsoft Windows Community forums reveals a workaround that one user has confirmed fixed their battery drain – just turn on your Bluetooth radio.
According to Arvind M., who posts in the thread as a MS employee, this is why the battery drain occurs when the Bluetooth radio is turned off:
Powercfg /sleepstudy reports any issues that are preventing the CPU from entering its lowest power state during Connected Standy (e.g. screen is off). In this case, the state of the specific UART controller is preventing the CPU entering its lowest power state. This UART happens to be connected to the Bluetooth radio on Surface. Unfortunately, the bug that causes this bad state occurs exactly when the Bluetooth radio is turned off in PC Settings.
It is reasonable to want to turn off the Bluetooth radio to conserve even more power, but be aware that the major contributor to drain on battery is overall system power use, not just an individual radio.
So in the interim switch your Bluetooth radio back on to maximize your battery life.
Arvind also confirms Microsoft is fully aware of this issue in the preview so hopefully that means a fix is in the RTM of Windows 8.1.
Personally I never noticed this drain because I keep my Bluetooth on at all times because I use a BT keyboard and mouse with my Surface RT when it is on my desktop.
Thanks to @MobileTechWorld for the tip on Twitter.