Just to put those words in context - I use Dell XPS 13 with Linux 4.7 and my power usage drops to 4.1 Watt discharge rate and holds an average of 6 W when I'm on average use.
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 :)