Recommended uninterupted power supply for DL4100

How did you decide on the size to use ie 900,1300 or 1500va. The only reference to power I have found is this on the WD DL4100 site.
Power Supply

Input voltage: 100V to 240V AC
Input frequency: 50 to 60 Hz
Output voltage: 19V DC, up to 6.32A

Do you know how that relates to va of the UPS.