Apparently the HD display comes with a huge price in battery life: http://www.cnet.com/products/dell-xps-13-2015-review/
The full HD (1080p) version is considered the low-res and according to Anandtech it lasts 15 hours of normal use, while the higher end model with a 3200x1800 screen lasts 10 hours of normal use (according to Anandtech), and 7 hours of video playback according to the CNet review.
Anandtech review: http://www.anandtech.com/show/6849/dell-xps-13-q1-2013-ultra...