Ubuntu 18.04 does not enable power saving for NVIDIA graphic cards by default. The card will be power-on, even if a secondary graphics chip is the active graphics card.
If you selected
sudo prime-select intel
to ensure that your Intel graphics card is active, then the NVIDIA card will not be disabled. You can use tools like bumblebee to manage the card. However, sometimes the build-in power options are sufficient. If you don’t want to switch to the NVIDIA card on demand, then bumblebee is not required.
Check your power settings on the graphics card. Check lspci to determine where to find your graphics card:
$ lspci | grep -i nvidia 01:00.0 VGA compatible controller: NVIDIA Corporation GP106M [GeForce GTX 1060 Mobile] (rev a1)
The 01:00.0 portion in the output shows which system part to you have to look at. For me, the sys file indicating the state of power control of the card can be read as follows:
cat "/sys/bus/pci/devices/0000:01:00.0/power/control" on
Replace the string after 0000: with your lspic output to determine the correct device. If the content of the file is on, then the card will not be powered off. Turn the setting to auto enable shutdown of the card in case it is not used.
sudo sh -c "echo auto /sys/bus/pci/devices/0000\:01\:00.0/power/control"
The card will turn off if not needed and the power consumption of your system will improve significantly.