Apple has optimized iOS 9 for better battery usage, and can extend battery life for an additional hour on iPhones under normal usage. Some trick is deployed to get more juices out of the battery. When the iPhone is placed facedown, iOS 9 uses the ambient light and proximity sensor to determine that the screen is not visible, and will stop the screen from turning on. Any incoming notification will not light up the screen.
iOS 9 also comes with a new Low Power Mode that can add up to three hours of battery life. When under Low Power Mode, iOS 9 reduces battery draining features. Network activity will be limited, automatic mail fetch is disabled, background app refresh and downloads are turned off and screen brightness are reduced. Low Power Mode also reduces the CPU speed so it drains less battery. Benchmark suggests the performance hit can be as high as 40% while on Low Power Mode.
iOS 9 will prompt you the option to turn on Low Power Mode when the battery level is at 20% and again at 10% if it is not on. If you missed the low battery alerts, you can switch on Low Power Mode under Settings > Battery.
For the first time in iOS, you can tell which app is taking most of the battery life. Visit Settings > Battery to view the usage details. Pay attention to app that is rarely used but is taking up the top spot.