To enable the power button on the start screen in Windows 8.1 (disabled by default on screens smaller than 8.5″), add either of the following registry keys:
For current user:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell
For all users on the device:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell
Value to add:
Launcher_ShowPowerButtonOnStartScreen, DWORD = 1
Note: if setting this value via Group Policy Editor, the HKCU setting will only work if applying to user objects and the HKLM will only apply to computer objects – so be sure to apply the correct setting to the correct OU.