I've got a system that has no battery, and a single user account.  The user account is logged in, but the screen is locked.

While the screen is locked, the notification area at the top of the screen shows three icons: network, audio, and power (that is, the "power symbol" of a circle with a vertical bar in the upper part of it, like [0]).
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Clicking in that area pulls down a menu/dialog-ish thing that has indicators/controls for networking and audio, but nothing for power.

When the user is not logged in, the notification area is the same, but the menu/dialog-ish thing contains a functional power button.

I don't think the notification area should show the power symbol if it's not going to offer power indicators or controls, so either the lockscreen's menu/dialog should offer a power button, or the notification area shouldn't show the power symbol.


[0] http://unicodepowersymbol.com/wp-content/uploads/2014/01/IEC5009_Standby_Symbol-fs8.png