One of the more significant new features in iOS 9 Apple mentioned during its keynote today is called "Low Power Mode." Similar to Power Reserve on the Apple Watch, this feature is designed to extend the battery life of your device.
First it should be noted that because of the under-the-hood improvements in iOS 9, Apple says battery life for the average user will increase by up to one hour. If you need more, Lower Power Mode gives you up to 3 additional hours.