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Better battery life is an Apple priority for iOS 9, and to that end the company has adopted extreme attention to detail, implementing things like face-down power restrictions, a Low Power mode, and a dedicated menu in the Settings app with more granular detail.
iOS 9 devices use light and proximity sensors to determine whether it's laying face-down, and stop the screen from turning on even when notifications come in. Displays are typically the most power-hungry component of any mobile device.
The Low Power mode can reduce consumption even further, mainly by limiting network activity. Email, for instance, is no longer fetched automatically, and background downloads and app refreshes are halted completely. Other measures include scaling back animations and screen brightness.
Apple is in fact promising that simply installing iOS 9 should give an iPhone 6 an extra hour of battery life. Turning on Low Power should add another three hours.
Partly because of that feature, the new Settings app moves battery options out from under a Usage submenu into a top-level section. The app can also track sources of battery usage on a narrower timeframe, recording since last wake, instead of just the past 24 hours or seven days.