By default GNOME (and hence Ubuntu 17.10 and later releases) doesn't show battery percentage in the notification area of the Panel. There are GNOME extensions (for example Battery Percentage, Battery Percentage and Time, Battery Percentage and Time Compact etc.) which do the job.
Is there a way to attain the goal without using any extension?
Open dconf Editor and navigate to /org/gnome/desktop/interface/show-battery-percentage. Then disable "Use default value" and select True in the "Custom value" field.
Alternatively just open Terminal and run
gsettings set org.gnome.desktop.interface show-battery-percentage true
Same settings can be found in (GNOME) Tweaks under Top Bar:
