HTC One (M8) for Windows
Battery Saver - HTC One (M8) for Windows
Battery Saver - HTC One® (M8) for Windows®
From the Start screen, swipe left to display the Apps list.
(located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap the 'Now until next charge'
When turned on, apps run only when you open them and email must be synched manually.
If enabled, select one of the following:
When battery is under 20%
Always (limits functionality)