Well, the fuse is wired in series with the PowerPlant, right, all current consumed by the PowerPlant is coming through the fuse. So with a 5Amp fuse, at 120 volts, the maximum wattage that the PowerPlant can continuously consume is 600 watts. Now there is a nuance to this. This is a slow-blow fuse. The UL standard for fuses allows peaks. From the LittleFuse website: “The fuse must open at 135% of rated current within one hour. It also must open at 200% of rated current within 2 minutes”. So the P15 could indeed consume a peak wattage of 1200, up to 2 minutes, before the fuse is required to blow. That is what must be meant in the documentation? I am sure your PowerPlant Engineers can explain in an instant.
Also, the PowerPlant stores energy in capacitors, so it can provide peaks higher then what it’s pulling from the net that very moment.
The breakers that come after the fuse might be for voltage peaks etc, or cap current before the actual fuse would.
Anyway, for my system and many others, this is all a bit theoretical because in reality, you might need a few hundred watts continuous at most, unless you are running massive class A amps or something else extraordinary. Plenty of headroom for most of us in that fine P15 of ours.