what are you looking for, the inline fuse? Start with a 100 amp inline fuse. and see what blows. I used to have a 100 amp inline fuse on my 1kw amps and never had issues. You could step it up to a 150 amp inline fuse.
the ap1500 does not have on-board fuses, so yes in-line near the battery. the site shows an included fuse holder, but doesn't indicate the amps of the fuse.