How much space does RSP require for backups

Hi All

I need to upload a client database via RSP.

However, it is failed and the error is there is not enough diskspace for the backup.

The drive where the backup is pointing to has 47.5gigs of space...and the database is 18gigs

Does it need more then double the space? Could it be user rights issue, although it does create a folder(date-time stamp folder)

Thank you

Jerusha