Windows 8.1 Preview version has been out and being tested actively for users as well as developer based issues. Surface RT owners with Windows 8.1 Preview version installed must expect some bugs to show in their operating systems. Microsoft has recently dealt with such an issue that caused a huge battery drain, the way the problem has been resolved is somewhat interesting.
Recently, a Surface RT user posted on the official Microsoft Support Forums that he was having a battery drain of 1-3 % each hour when his tablet having Windows 8.1 Preview was under Connected Standby mode. He further explained the issue that the major reason of the battery going low so quickly is affected by Microsoft UART V2 Driver (_SB.UAR3) while using Powercfg /sleepstudy option on the tablet.
It is in the reply to his post where the interesting part lies. A Microsoft Support person suggested the tablet owner to switch the Bluetooth wireless radio on. This simple step indeed solved the battery issue though apparently this step would lead to the battery being used more.
It is unclear to most of us how this could have been the answer to such a problem but Microsoft explains it all:
Powercfg /sleepstudy reports any issues that are preventing the CPU from entering its lowest power state during Connected Standby (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.
According to the support member, this is not a new issue rather a previously known one for Surface RT running Windows 8.1 Preview. It is expected to be completely resolved prior to the release to manufacture (RTM) phase due this month.