Hi,
Assuming you have only the public schema in use per database (usually the case). In each database run the following SQL:
CREATE ROLE backup WITH LOGIN;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO backup;
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO backup;
After that, adjust the file PostgreSQL configuration file pg_hba.conf to allow appropriate access for newly create ROLE backup.
Yours,
Ville