There were 2 available PSU for the 8940, 360w and 500W. Either one will be more than enough for a GT 1030.
Just bought the XPS 8940 in 2022, and want to upgrade the video card to: Gigabyte GeForce GT 1030 GV-N1030D5-2GL Will the existing power supply work or do I need to upgrade that as well?   
XPS 8940, GeForce GT 1030, new PSU required?