BatteryEater Pro tends to measure worst case scenarios, in that it doesn't really take into consideration power saving features, instead loading up the system until it dies out. It runs a spinning graphic constantly until the battery dies. We keep our test machines with Wi-Fi on, and screen brightness hovering at 50% for the life of the test.
Given that the XPS 15z wasn't the beast of all beasts in the gaming benchmark category, and that NVIDIA's Optimus was employed here, we expected the 8-cell battery to power through quite a few hours for a thin-and-light, unlike most gaming notebooks. BatteryEater Pro tore through those hopes, mostly because its animation forces the discrete GPU to remain on. However, in general use, we easily got 3.5 hours of use before it dropped itself into sleep, using a mix of discrete & Optimus GPU, and keeping the screen on the entire time with brightness between 40% and 60%. In other words, if you press this thing hard with games, you'll be lucky to get 2 hours. If you take it easy, you could see 4-5 hours depending on usage. It's nice to have that range for days where you simply need an integrated GPU to handle Web browsing and document processing.