Default password for postgreSQL

There is no default password on Windows - the install will ask you during the wizard to create one. If you forgot or don't remember you can:

Go to Control Panel -> Administrative Tools -> Computer Management. There under "Local Users and Groups" you can see all users created for your system. Remove "postgres" and reinstall PostgreSQL

On a default Windows installation you must specify a database name and you have the option to change the hostname, the username, and the password.

Defaults are: Host name : localhost, Database name : <wasyourchoice>, User name : openpg, Password : openpgpwd

Though it's late, putting it across :

By Default, the user is 'postgres' and the password is the one which you enter while installing the database. (Version 11,12 I have tested).

Issuing the command

psql 

from cmd in windows will ask for the password of User (Logged in) which might not be available. So you can try login with postgres user :

psql -U postgres

and enter the password used while installing.

Or create a user with login permissions using PgAdmin tool.