SQL command to determine total data backed up for the previous 24 hours.
SELECT activity , CAST(sum(bytes/1024/1024/1024) AS decimal(8,2)) AS "Total GB" FROM summary WHERE activity='BACKUP' AND start_time>=current_timestamp - 24 hours GROUP BY activity

If you want total data backed up by node then use this select statement:
SELECT entity as "Node", CAST(sum(bytes/1024/1024/1024) AS decimal(8,2)) AS "Total GB" FROM summary WHERE activity='BACKUP' AND start_time>=current_timestamp - 24 hours GROUP BY entity

Source: http://momo66.wordpress.com/category/tsm/backups/

Cheers
Ryan Partington