Hi,
We don't know what do you want to backup. Backup the whole drive or just user's data therefore we can't tell. My machine without user's data/files still uses around 90Gb:
This is one of my test machines, probably 40Gb is just test programs therefore I stll need 50Gb or so to backup if I wish to backup full drive. External HDD's are very cheap now, you can back up a whole drive to it.
Regards.