This has a maximum continuous output of 70 amps and momentary of 85 amps in a nominal 12 volt system. Depending on how you set the output voltage the maximum Watts would be in the region of 1100 Watts (13 volts x 85 amps)
If your source battery was at exactly 24 volts 1100 Watts would be 46 Amps. Since that is the absolute max draw a 50 amp fuse would probably be appropriate.