THIS article covers practicaly everything you need to know
To see if the author of the above link was right about his approximations in "Table 2" I tested my cooling fan, it measured 5.25 amps or 63 watts when running so his suggestion of 60 watts is in the right ball park.
Not having a running bike I can't test the power consumption of the TCI ignition so I'll just have to use his suggestion of 50 watts
Right, an example
XTZ660 (3YF)
Manual states alternator output at 5000rpm to be 14 volts with 24.5 amps
Power = volts x amps so 14x24.5= 343 watts at 5000rpm
But that is more or less peak power, looking at the graph in the manual it shows 23 amps at 3000rpm.
Assuming it is putting out 14 volts, power is less, 14x23= 322 watts
So real world alternator output power is between 322 and 343 watts
Now take away any constant loads the bike needs to run (items that may be on for a long time)
Ignition = 50watts
Headlight = 60watts
Tail light = 5watts
Instrument lights = 2x1.7 = 3.4 watts
Which leaves 203.6 to 224.6 watts for accessories.....
Well not exactly,
If you push the limits with accessories it leaves less power to re-charge the battery, charge lost to the starter motor, indicators, horn, brake light & cooling fan (Note I don't ride with the brake light on or fan running all the time unlike the author of the link, we must have different riding styles
)CDI ignitions are self generating so don't need to be subtracted from your constant loads
Dirty connectors & corroded wire add to the load
Hope this helps with your accessorising
Rick