Q: I assume that the boot up process on the iPhone uses more energy than when the phone is in sleep or Airplane mode. Let’s say you need to periodically check your iPhone for email or voicemail, but don’t need to know immediately if either of these things happen. At what point does it pay to turn your iPhone off and on, as opposed to putting it into Airplane/sleep mode, to maximize your battery life?
A: For the most part, leaving the iPhone in Airplane Mode and otherwise sleeping with the screen off will significantly reduce battery drain. The main source of power consumption on the iPhone is the cellular and Wi-Fi radios and the iPhone screen itself.
While turning the iPhone off entirely will obviously consume even less battery, the difference in battery consumption compared to sleeping the iPhone in Airplane mode is relatively small. Further, although the startup process will consume some additional power, this is also fairly minimal, although obviously higher than simply putting the iPhone to sleep in Airplane mode.
Another important consideration is the time it takes to wait for your iPhone to boot back up after powering it off. Considering that powering off the iPhone entirely doesn’t result in any significant reduction in power consumption versus simply putting it to sleep in Airplane mode, it’s probably not worth the additional wait time when you want to actually use your iPhone.
Keep in mind that the iPhone can easily get more than 24 hours of audio playback time on a full charge, even with the cellular and Wi-Fi radios turned on. Further, our tests with the iPhone 3G have actually shown playback time in excess of 29 hours while in Airplane mode. Without audio playback, the iPhone could remain sleeping in normal mode for a couple of days, and would last in Airplane mode even longer.
Note that you should always return your iPhone to the home screen before putting it to sleep.