Apple released “Energy Efficiency Guide for iOS Apps” few days ago when releasing Xcode 7 beta 4. Note about this document is in the change log.
Not all people read change log so I would like to mention this document here.
I noticed that recently Apple cares about energy more than usually. I read this new document and would like to recommend to read it for everyone. You can find there a lot of system-level informations and tips how to use energy efficiently.
Document describes how system works e.g. with WiFi antenna or 3G/LTE or how timers are used by the system, how to deal with disk I/O operations or networking stuff, etc.