Battery Usage on Your iPhone with iOS 9
You count on the iPhone battery getting you through the workday and well beyond. The controls under the Battery setting can help you monitor how much juice is left and give you a chance to get something extra out of the battery.
Low power mode: When it’s critical to keep the battery going but you’re unable to charge your phone for a while, consider flipping on the Lower Power Mode switch. Upon doing so, the phone temporarily cuts down on power consumption by reducing or disabling various tasks, including fetching mail in the background, refreshing apps, and downloading stuff automatically. Some visual effects are also eliminated.
Battery percentage: You can turn on a gauge that reports how much of your battery is charged in percentage terms. The percentage appears just to the left of the battery gauge at the upper-right corner of the iPhone. You also see the battery gauge in Notification Center under the Today view.
Battery usage: You see the proportion of battery use by each app in the last 24 hours expressed in percentage terms.
Time since last full charge: This information is indicated in days and hours, for the time when the iPhone has been unlocked and in use and also when it has been in standby mode.