One of the reasons we've always worried about using the open Ogg and WebM formats on iPhones and iPads is that we don't get to make use of the hardware MP4/H.264 codec... using the main CPU cores is presumed to drain the battery faster.
I've done a first-pass test measuring energy usage of native-code WebM/Ogg playback using the new energy reporting in the Xcode 7 / iOS 9 beta:
https://brionv.com/log/2015/06/19/webm-and-ogg-energy-usage-on-ios-9-beta-wi...
The good news is that playback of buffered data at my target resolutions (360p for 32-bit, 720p for 64-bit) is barely under the "Low" mark on the energy drain meter! :D
The bad news is that H.264 playback with the native widget reports post-buffer-download energy drain even lower, at the "Zero" mark... if you can believe that!
(In both cases, reported drain is significantly higher during network download, at least on my fast wifi.)
But "Low" sounds pretty good... If folks would like to see more concrete measures, I can rig up my test to run continuously until the battery runs out and time it.
-- brion