How much battery energy does each app use?
When troubleshooting excessive battery drain it is extremely helpful to determine which app or apps are the heaviest users of energy. iOS gives you an easy way to do this - just tap on the Settings app, then scroll down to Battery and tap on it. This app, among other things, will list the apps that are the heaviest users of energy. (Note that the phone must have run for at least 24 hours without powering off for the display to update.) In the initial view it will show a percentage; this is the percentage of total battery usage over the previous 24 hours that each app has used. Note that this is NOT the actual battery usage; it is an amount of an amount - that is, if your phone's battery has gone from 100% to 50% in the past 24 hours, and an app shows 23%, that does not mean that the app has used 23% of total battery capacity; rather, it is 23% of the 50% , or 11.5% of the total battery capacity. The reason is that you may have charged it more than once in those 24 hours, so it would be meaningless to show it as a percentage of battery capacity. There's also a view that shows battery usage by each app over 7 days.
The more interesting view is the second view; tap on any displayed app and the view will switch to a breakdown of on-screen and background usage. It will look like this:
This view shows how much time you were actually using the app and how much time the app was running in background. Well-behaved apps should have a very small background value; in this specific example the Tile app ran for 2 hours in background over the previous 24. That's 8%, which is a huge amount of energy. The NYTimes, at 6 minutes background, or 0.4%, is more typical.
Where this comes in handy is if your phone is using a lot of energy, and needs to be recharged frequently. Usually that is caused by a "misbehaving" app. Using this simple approach you can see which app it is, and take remedial action.
Another use ties in to the practice of killing apps in the multitasking screen. You can see how little killing apps actually saves.
