Also worth noting is that NVME ssd power management is flat out unsupported resulting in additional 3W power draw on DELL XPS 13 pcs out of the 6W that's currently possible with stable power options enabled.
That means that I usually have around 7-8 hours of battery on my average use, and if I'm in a battery-sensitive environment like a long flight, I can reduce screen brightness and turn off wifi and get up to 14 hours of battery.
Right at this moment I have Fx with 2 windows, 10 tabs each, half-brightness, wifi on, and I'm at 5.78 W, 87% battery with 9h 28m left estimated.
I believe Windows may have even better, and I see my NVMe not dropping to the lowest power saving state, but it's not that bad IMO :)
I do have a Surface pro 4 with a skylake chip in it and even Microsoft appears to have struggled with power management on that chip. The Skylake NUCs have also been plagued by multiple issues. I briefly looked into getting one, but the issues scared me away.