Select sum of an array column in PostgreSQL

SELECT id, (SELECT SUM(s) FROM UNNEST(monthly_usage) s) as total_usage from users;

In several ways to add up the values of the array, the form I always use is:

WITH X AS(
    SELECT unnest(ARRAY[1, 5, 0, 12, 1, 0, 30, 20, 8, 3, 15, 15, 20, 8]) AS VALOR
)
SELECT SUM(VALOR) FROM X

Result:

For more information https://www.postgresql.org/docs/9.1/queries-with.html

This generalization and reformatting Dmitry's answer helps me understand how it works:

SELECT
  sum(a) AS total
FROM
  (
    SELECT
      unnest(array [1,2,3]) AS a
  ) AS b

Result:

total
 6

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