About 5 km altitude. But for both KiteGen and Rotating Reel carousels the diameter of the ground rotor is important due to the efficiency requirement.This is still more detailed on the linked post below with two references:
KiteGen vs Rotating Reel carousels . I mention:
So the possible kite altitude for an efficient conversion depends also on the diameter of the carousel, and that in a similar way for both KiteGen and Rotating Reel carousels. See the page 14 of http://www.kitegen.com/pdf/dossierkitegenen.pdf : (probably average) operative kite altitude 0.75 km for a machine’s diameter of 1.13 km, then in the end an operative kite altitude 10 km for a machine’s diameter of 25 km.
Some other difficulties can occur, above all when the targeted kite altitude is 10 km as mentioned on the document.