Continue backup

I am attempting to create a gui for dar backup. The gui will cater to
people who don't have much idea about backups, basically click backup to
backup the data. The backup tasks will run unattended. This means that a
user might shut down the pc or there might be a power cut while the backup
is running. Since the target computer is unknown it is also not possible to
separate the files based on directories. Different users will have
different directory structure.

How well does dar deal with stuff like this? How can I continue the backup
where it left off ?