With a 580 watt alternator you have about 48 amps full load. We have a 14 amp/hour battery which will run 168 watts for one hour. The user manual says the standard socket is good for 5 amps or 60 watts. This is for 12 volts at 100% conversion which is, of course, not correct.
What does this all mean with regards to your question? 228 watts is required to keep the battery charged and the socket running at its rating so what about the other 352 watts capacity of the alternator? A lot has to do with the output of the alternator vs engine rpm but I'd say its good enough for adding some small loads over the socket load. Another 60 watt load should be safe.
You will see right away by voltage, and you know for sure that you can add 60 watts if you don't use the socket.