It seems that Microsoft has missed out something causing some issues with the battery life to Surface RT users. From the information, it affects those that upgrade from Windows 8.1 RT Preview to Windows RT 8.1 only. If you did not see or feel any problem in your battery life, don’t do anything.
Here is the explanation from Microsoft:
During the update, the wireless adapter power policy isn’t migrated. Instead, the power policy is set to a default value that consumes more power both during use and in the connected standby state.
And here is the steps to restore this policy (disclaimer: I copied everything from Microsoft Surface RT site, so follow it at your own discreet. I shall not be responsible for any “mishap” or issues arising from it.)
To restore the wireless adapter power policy to the correct settings, open an administrator command prompt:
Step 1: Swipe in from the right edge of the screen, and then tap Search. (If you’re using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, and then click Search.)
Step 2: In the search box, enter command prompt.
Step 3: Touch and hold (or right-click) Command Prompt to bring up the context menu. Tap or click Run as administrator.
Step 4: On the User Account Control dialog box, tap or click Yes.
Step 5: At the Administrator: Command Prompt, enter the following:
powercfg -setdcvalueindex SCHEME_CURRENT 19cbb8fa-5279-450e-9fac-8a3d5fedd0c1 12bbebe6-58d6-4636-95bb-3217ef867c1a 3
Step 6: Then enter powercfg -setactive scheme_current
Microsoft actually added this in their Surface RT under the support section. Maybe they realize that if they put it as Knowledge Based, it might not be as useful.
Click Here to Check Out the Surface RT Support Site
Let me know if you have any success 
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(source)