Thank you for the replies.
On CentOS 7 Linux with pgdg 9.5.3 the lower() method works and I have:
# \l
List of databases
Name | Owner | Encoding | Collate | Ctype |
-----------+----------+----------+-------------+-------------+
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
| | | | |
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
| | | | |
words | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |