It's easy to compile a list of iPhone power-saving tips, but I'll tell you now that putting them to the test to find out which work and which don't is tedious and time-consuming work. But after several weeks of real-world testing I've found what I believe to be the best iPhone battery saving tips, along with a handful that are just not worth bothering with. For these tests I used three different iPhones - an iPhone 5s, iPhone 6 Plus, and the new iPhone 6s - and all three were running the latest iOS 9.2.1 update. All the handsets are fully functional and battery life appeared to be normal. Each iPhone was configured in a similar -- albeit not identical -- fashion. I then applied one of the many battery saving tweaks, then they were used normally throughout the day (email, iMessage, Facebook, Twitter, web browsing, some calls and such) for several weeks while measuring battery consumption at fixed intervals. What I was looking for wasn't how long I could get out of the battery, instead I wanted to find out which tips extended the battery life the most. And spoilers; many of the battery saving tips that I tried seem to have a negligible effect on battery life.
Note: My listing is based on real world testing. However, I'm fully aware that your real world might be different to mine, and that as such your mileage may vary. For example, if you do a lot of gaming on your handset, or use the camera a lot, then this will have a dramatic impact on battery consumption, and the tips below might not help you as much as they'll help someone who uses their iPhone for email, Facebook, Twitter and web browsing.
Activate Low Power Mode
Sometimes the simplest things are the best. This feature automatically tweaks multiple settings to save battery power, disabling automatic fetching of new mail, reducing screen brightness, and automatically locking the phone and powering off its display more quickly. Apps are also able to detect when low power mode is enabled and can help to further increase battery life by disabling animations and other battery-heavy features. And the great thing about this is that you're prompted to turn it on when the battery hits 20 percent and then again at the 10 percent mark. You can manually activate Low Power Mode by tapping Settings>Battery>Low Power Mode. If you do this when you have a fair chunk of battery left - say above 50 percent - it's possible to get as much as a few hours of extra usage. The downside is that the iPhone does run noticeably slower, and this can put a dent in productivity.
Turn down the brightness
Another simple tip that makes a night and day different (pun intended) to battery life is simply turning the brightness down. Just flick the Control Center tab from the bottom of the screen to access the brightness setting. I find that I can happily keep my iPhone at a low brightness setting almost all the time, bright sunshine outdoors being the main exception, but since I live in the UK, that's not a thing I have to deal with often.
Turn off Wi-Fi when not in use
Wi-Fi really burns through battery life, and it seems worse when it's away from your home or office and actively searching for Wi-Fi it can connect to. If you don't need it, turn it off.
Turn off cellular data
This is for people who find themselves in areas where there's poor cellular coverage. One of the biggest drains on the battery I found was keeping the iPhone in a location where there was no cellular coverage. It continually looks for a cellular connection, burning through battery life in the process. Again, if you don't need it, turn it off. It puts the brakes on notifications and background apps that all put pressure on battery life. If you're out of cell coverage doing this can easily give you a couple of hour of extra battery life.
This is the nuclear option, and stops all wireless connectivity. The process of searching for a connection, whether that be cellular, Wi-Fi, or Bluetooth, seems to put a severe dent in battery life, and the easiest way to stop this is to flip the handset into Airplane Mode (flip the Control Center screen up from the bottom and tap the airplane icon). If you need Wi-Fi or Bluetooth then you can still turn these on separately (again, from the Control Center screen - just tap on them). With no connection to the Internet or cellular, your iPhone essentially becomes a brick, but at least you can turn off Airplane Mode selectively when you need it, saving a considerable amount of battery life.
The following did have a small effect on battery life, but they were nothing to write home about.
Disabling the Home screen visual effects
Turning off AirDrop
Lowering the speaker volume
Five of the worst
I discovered that many of the popular tips seem to make no noticeable difference on battery life. Here are five of the most useless battery saving tips I tried.
Turning off Bluetooth (the newer handsets have Bluetooth Low Energy so power consumption when not in use is very low).
Closing apps (the only time this would make a difference is if a background app was going crazy, like the Facebook app was acting back when iOS 9 was launched).
Turning off the auto time zone feature.
Turning off vibrate for calls and texts.
Turning off Spotlight search.
Why worry about battery life?
I think that life's too short to worry about my iPhone's battery life, which is why I carry around with me a portable battery pack. These days power packs are so small (here's the Anker PowerCore Mini that's marketed as being "lipstick sized") that you can pop them in a pocket, and yet they pack enough of a punch to add hours to your iPhone's battery life. What I'm doing from now on is carrying a spare battery pack with me (the Anker PowerCore Mini is perfect), and if I think that's not going to be enough to get me through the day I activate Low Power Mode. Fiddling with the iPhone's settings in order to try to get a bit more battery life is, in my opinion, tedious and there's a risk that I'll forget to undo the settings later, so from now on I won't be doing it.