Dell's own self reporting on their website, I presume. I got an XPS15 a couple of months ago and I have no idea what crack Dell smoked to get that number. I don't have a problem with the battery life personally since I'm coming off a very old laptop which has about a 6 minute lifetime, so the 4-5 hours I can get is a breath of fresh air, plus I tweaked the battery to the "mostly plugged in all the time" setting which I think works by lowering the max charge limit to 80%, so most people will probably get more than I do (at least at first). But if you're used to getting 10 hours from an existing system you may be disappointed.