I turned off my Nvidia GPU with prime-select today after a few weeks running dual-screen on GPU. The laptop is a Thinkpad P51 / Intel Skylake Xeon / Nvidia Optimus Quadro M2000 with Ubuntu 18.04 up to date, Nvidia driver is 390.
The first thing I noticed after rebooting was the HDMI exteral screen was still on, which is not possible since the HDMI port is wired to the GPU and can't work with the GPU off. So, nvidia-settings and nvidia-prime show that the active GPU is Intel, but in fact Nvidia is on.
Looking at the powertop output, my computer runs at 19-22 W idle and burns its battery in 4h, which is typical of the Nvidia GPU (on a fresh install, I did tests with Intel at around 9 W idle and it lasted almost 10h on battery).
Of course, I did all the
apt autoremove nvidia* dance twice, reinstalled xorg etc. My GPU is on whatever I do. How can I force it off ?