Find total size of all databases in PostgreSQL cluster

SELECT database, size FROM (
  SELECT datname AS database,
         pg_size_pretty(pg_database_size(datname)) AS size,
         0 AS sort_order
  FROM pg_database
  UNION ALL
  SELECT 'TOTAL',
         pg_size_pretty(SUM(pg_database_size(datname))),
         1
  FROM pg_database
) AS sub
ORDER BY sort_order, size DESC;

Example output:

             database             |  size
----------------------------------+---------
 mydb01                           | 7819 kB
 mydb02                           | 7795 kB
 postgres                         | 7739 kB
 template0                        | 7731 kB
 template1                        | 7715 kB
 proddb01                         | 76 GB
 proddb02                         | 2971 GB
 proddb03                         | 22 GB
 warehouse01                      | 11 TB
 testdb01                         | 106 MB
 TOTAL                            | 14 TB
(11 rows)