[0xmohit]

One might have sudo privileges configured via /etc/sudoers (It's the default on AWS EC2 instances!)

[icebraining]

Yes, but you still need to call "sudo poweroff", not just "poweroff".

[avhon1]

See user chainsaw10. Anecdotally, on my vanilla Arch systems, I can run "poweroff" without being root, and without sudo, and the computers will end all processes and turn off.