gistfile1.sql

SELECT  id,
  sum(hours) AS totalhours
  FROM mytable
  GROUP BY id
  HAVING sum(hours) > 50;

If you really can't be bothered to write sum() twice, you could consider
a two-level SELECT:

SELECT * FROM
  (SELECT  id,
    sum(hours) AS totalhours
   FROM mytable
   GROUP BY id) ss
WHERE totalhours > 50;