[Unit]
Description=Backup Files

[Service]
Environment=SOURCE=/path/to/source
Environment=DESTINATION=user@host:/path/to/destination
Environment=OPTIONS="-HAax --del"
Environment=EXCLUDES="--exclude path1 --exclude path2"
Type=simple
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/bin/sh -c "/usr/bin/rsync ${OPTIONS} ${EXCLUDES} '${SOURCE}/' '${DESTINATION}/'"