Using VLC and the 1080p .ogg file, I saw about five hours on the QHD+ XPS 13, another half-hour so on the MacBook Air 11, and just over seven hours on the 1080p version of the XPS 13. It's pretty clear that with a lower-resolution file and using Windows 8.1's built-in Metro video player, the XPS units can come close to the rated video run time. There should also be a healthy bump up in run time for the MacBook Air using iTunes.
So the MacBook Air 11 is better in run time, right? Sometimes. When I ran my original review, I actually didn't use the shorter .ogg file. I used our encoding test file: a 31GB MKV file that's high bit-rate and 1080p.
That file is a bear, and we can see all the laptops taking a far bigger hit in battery life--including the MacBook Air 11, which suddenly falls behind the XPS 13 QHD+. This could be because the Haswell CPU in the MacBook Air 11 worked harder to decode the file than the Broadwell CPU in the XPS 13, or possibly something to do with the M.2 drive in the MacBook Air, which is PCIe-based and far faster than the M.2 SATA in the Dells.
No one gets bragging rights
After trying to wrangle cross-platform battery testing and seeing how many variables can go into the tests, I've decided that the best we can say is battery life on X platform is better for Y task. So, I can say quite comfortably that playing 1080p MKV files using VLC, the XPS 13 gives you better battery life.
I also believe that unless you're saying what exactly you're testing and how you're testing it, it's not very easy to compare the two platforms on pure battery life. I also don't think it's valid to try to compare platform Y against platform X by seat-of-the-pants testing of "using it." That's just too hard to replicate in a manner that's actually useful beyond trolling each other in a forum.
Maybe the best way to settle this discussion next time is to say your mileage may vary.
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