Frustratingly, there is no easy answer to this question. In iOS 8 Apple began to allow users to see which apps are the biggest battery drainers, plus Apple offers the ability to discover the percentage of battery power remaining, and you see how long your phone has been running without a charge, and how much of that time you have been using the iPhone, but if can’t tell you how many hours you have left.
This is probably because the amount of battery life remaining is entirely dependent on what you are planning to do with your iPhone. If Apple told you to expect two hours and then you ran a movie on full blast you would probably run out of battery before the movie ended.
However, there are third-party apps that can give you some guidance about how much battery life is remaining.
One such app is BatteryDoctor (previously BatterySaver) from KS Mobile (buy it from the App Store here). This tool offers a broad range of system tweaks, with a particular focus on saving battery life.
The app’s main screen shows an estimate for how much battery life is left, based on what’s running in the background, and your current system settings. If you perform some of the tweaks suggested by the app, you’ll see this number creep up.
For example, when we switched on Airplane Mode we gained about an hour of battery life – the battery life remaining changed from 8hrs, 17mins, to 9hs, 21mins.
Tap on Optimize to see a breakdown of how much longer your iPhone could last if you shut down certain services, such as disabling Wi-Fi or GPS, or reducing brightness.
Tap on Remaining for details of how much time you have remaining to do certain tasks, web browsing on Wi-Fi, or web browsing on 3G, talk time, video playback, photo taking, and more.
The app Battery Doctor will also alert you to any app being used, which might prove surprising, as in our case it highlighted Facebook which was open, as were many other apps, and yet Facebook appeared to be the only one consuming power in the background (despite the fact that we had turned off Location Services and background updates for that app).