20A or 1kW is not bad for that type of 10ft machine and is realistically all you can expect the thing to stand.
The only real issue is the wind speed at which you get it. As you have no idea of wind speed then there is no way of knowing.
"i mean i have seen 16-17 amps in strong gusts and on one ocasion was able to see it produce 21 amps. but it seems like it takes a lot of wind to get there."
That may imply that you are in a poor wind area ( as most of us are). There sometimes isn't much energy in gusts. If you get sustained good wind it may perform a lot better.
Unless you can devise a way of measuring prop speed it is hard to know where you are working.
It sounds as though the cut in is plenty low enough and with little line resistance I would expect you to stall and it does sound a bit that way now but you should have enough line resistance to avoid it I would have thought.
See if you can measure prop speed from the frequency of the incoming ac, that will give a better idea of where you are working.
You may get some idea looking at it, if it is slow and lazy and very quiet then you could try adding an ohm or two in the dc positive line and see if it livens up.
If it is already fast and noisy then you may have more line loss than desirable .
My 10ft machine does 50A quite easily into a 28v battery ( not a typo) in winds around 25 mph. That would be something like 25A for your 48v.
You won't get that sort of performance as my alternator is far better matched in the 25mph wind region and uses a boost converter to deal with the low winds.
I am inclined to think that you are not far off target but without reliable wind data it is nothing but a guess. If you average 200W on most days with a 10ft machine you are doing ok.
Flux