Battery life is one of the aspects of the XPS 13 that Dell is making bold claims about. Calling the battery performance of this laptop "industry leading", they say the 1080p model will last up to 15 hours in some sort of unspecified workload. The QHD+ model gets just 11 hours of life, although this could be expected anyway from the increased resolution (177% larger) and addition of a touchscreen. Again, I had the 1080p model to review.
I was immediately skeptical of Dell's 15 hour battery claims because that's well over what I've experienced on similar devices in the past. The MacBook Air 13-inch (for example) is a great performer in terms of battery life, but it doesn't really achieve its claimed 12 hours of life in practice. Anything above ten hours is pretty optimistic in general, and considering many devices on the market barely scrape past eight hours in practice, claiming the XPS 13 achieves nearly double this sounds very far fetched.
However we are dealing with new silicon from Intel that specifically focuses on energy efficiency improvements. The 15W TDP of the Core i5-5200U inside the XPS 13 is nothing new, but improvements to lower power states should see increased battery life in everyday usage. The XPS 13 also includes a sizable 52 Wh battery; large considering the footprint of this compact laptop.
While I didn't ever achieve 15 hours of battery life in my practical use of the Dell XPS 13, I did achieve life much higher than other laptops I've tested. The XPS 13 easily lasted a full day of work off the charge, with plenty of room to spare, and at a reasonable brightness (around 50%, as this is a bright display). During this time I was mostly editing Word and Excel files, browsing the web and watching a few streamed videos, so nothing hugely CPU or GPU intensive, but there was frequent use of Wi-Fi and the internet.
In general day to day usage I'd expect battery life anywhere from 11-13 hours in similar conditions: 50% brightness, browsing the web and editing documents. This is an excellent result, and I could easily see someone getting 8+ hours doing a few more CPU intensive tasks (like photo editing) throughout the day. If you really stretch the battery (Wi-Fi off, low brightness), you might get to Dell's claimed 15 hours of battery life.
Although I didn't test the QHD+ XPS 13, I'd expect results to be adjusted according to Dell's rated battery life. They state the battery life of the higher resolution model is 26% lower, so this would realistically correspond to 8-10 hours of typical use. I'd definitely sacrifice the higher resolution for 3 hours of extra life, even though the battery performance from the QHD+ model should be pretty good anyway, but that's just me.
In our battery benchmark tests, the Dell XPS 13 performs well, though again it doesn't reach 15 hours of life. The Wi-Fi browsing test is especially intensive as it's constantly loading webpages, so with more moderate usage you can expect longer than 8 hours of life.