Hello obersonn -- I haven't tried using the D-Lite RX 4/4 kit with an external power supply myself, but Elinchrom's specification list indicates that the maximum power draw during normal running is 150 Watts at 230V/50Hz, which goes up to 370 Watts during the charge phase. An external power supply spec list should include its max power output as well as its capacity, which should give you the information you need. (It's going to be fuzzy maths, since it's hard to calculate how many charge phases you'd go through in a session, the effect of temperature on the charge capacity, etc.) Good luck in your research!