That's a good question and I don't have an answer. The best way to put it though is how many AMPS the stator and Rectifier/Regulator can put out. The stator is 3 phase so hard to measure directly. The designers of the system would know what it's intended for but they don't seem to be on this board.
I could do the following except I live in an apartment building and don't have a garage. My patio is an echo chamber so the manger gets pissed if I run the engine there.
Someone take a clip on current meter and measure the output of the regulator while the engine is running at idle (*). Then the load on the 12 volt system after needs to be increased somehow until the voltage starts to drop. This can be done with big resistors or something else that draws a steady current. The point the voltage drop is the maximum current draw. One should really stay a bit lower than this to avoid overheating, etc. This current times the voltage will give you the maximum watt output.
* I say at idle because lighting and and heating elements are most likely to be on all the time. The stator can give more power at higher RPM's. If there's not enough power at idle you could add a switch and only turn them on when you're on the highway.