Hi,
- Are you doing file backup or creating a system image backup?
I would suggest you to refer this link and check if it helps:
http://windows.microsoft.com/en-US/windows7/Set-up-or-change-automatic-backup-settings
For more information, refer to this section” How much storage space do I need to backup files” from the below link:
http://windows.microsoft.com/en-US/windows7/Back-up-and-restore-frequently-asked-questions
Azam – Support engineer.