The Windows 8.1 Preview has been in testing for a while now, and some users who enjoy trying out new things have installed this latest system on their Surface RT tablets. Since it is a test version, encountering issues is unavoidable. Some users reported that after upgrading to Windows 8.1, the battery drained much faster than usual.
In the Microsoft Support Forum, a Surface RT user mentioned that after installing the Windows 8.1 Preview, their tablet would lose 2%-3% of its battery life per hour even when in standby mode. The main culprit behind this excessive power consumption was identified as a driver: the Microsoft UART V2 Driver (\_SB.UAR3).
Microsoft's technical support staff suggested that turning on Bluetooth on the Surface RT could resolve this issue. Following this advice, the user enabled the wireless Bluetooth on their tablet and found that the problem of rapid battery drain was indeed resolved.
Microsoft also explained the reason: in connected standby mode (for example, when the screen is turned off), Powerfg/sleepstudy reports all issues that prevent the CPU from entering a low-power state. A specific UART controller prevents the CPU from entering this low-power state, and this UART is connected via Bluetooth. If you disable Bluetooth in the PC settings, it causes the system to continuously consume power.
Microsoft is already aware of this issue and will fully resolve it in the Windows 8.1 RTM version.