Hmm... 8 watts sounds extremely high to me. My Broadwell laptop uses less than 3 watts when idle, and most of that is the display. Even my old Ivy Bridge MacBook Air from 2012 idles at 5W. Is there some peripheral responsible for the higher idle on your system? Maybe a hard disk or dedicated GPU? I'm curious about what's causing the difference.
I installed tlp and system went down to 5.5W idle with normal programs open. On fresh boot as low as 4.9W.
Turning the screen off shaves 2.5W, and disabling the wireless card saves ~1W.
I suspect the SSD + HDD are making part of the difference.