Help with authentication on Debain/Ubuntu installation

I am trying to set up to do some work with pg_dump, and I would like to be
able to connect from my normal user to do this. This is on a Ubunt 18.04
installation. I have added the follwing to pg_hba.conf:

host all all 0.0.0.0/0 md5
hostssl all all 0.0.0.0/0 md5

But when I try to login like this:

psql stan postgres -W

I get the following error, and yes, I set postgres'es password to something
I know:

tan(at)stantest:~$ psql stan postgres -W
Password:
psql: error: could not connect to server: FATAL: Peer authentication
failed for user "postgres"
stan(at)stantest:~$

What am I doing wrong?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin