From the Start screen, swipe left to display the apps list.
Alternatively, tap All apps (located in the lower-right; may require scrolling to the bottom).
Tap Battery saver.
Tap the "Battery Saver" On/Off switch.
When turned on, apps run only when you open them and email must be synched manually.
If enabled, tap Battery saver settings.
Tap the Battery saver check box to set the percentage at which point the Battery saver automatically turns on.
Enabled when a check mark is present.
If check box is unchecked, the default setting at which Battery saver activates is set at 20%.
If desired, tap Add an app to allow some apps to stay in use while Battery saver is running.