[SV650rules]

The rectifier bit ( 3 phase ) produces heat due to voltage drop across diodes, which is a about 1volt per diode for silicon diode, this is independent of current passing through the diode, so heat produced stays pretty much constant. The output of the rectifier is unidirectional sine waves, not dc, because it has a ripple. The alternator output voltage rises as the load current drops, the regulators job is to try to keep voltage at around 14 volts it does this by drawing current from alternator, the resistance of alternator coils drops the output voltage, this clips the unidirectional sine waves and reduces ripple and produce a low ripple unidirectional voltage. As the alternator rotation speed increase the regulator has to work harder and absorb more current to keep voltage stable, this current causes more heat ( watts ) which is proportional to square of current. The resistance of alternator coils is constant so it needs a constant amount of current drawn to keep output voltage at required value, the more of this current is being drawn by devices ( bulbs, heated grips etc. ) the less current the regulator has to absorb. It is pretty crude but works.

As already posted a car system controls alternator voltage output and keeps it stable against load by reducing excitation of rotor coil, some bikes have this system but SV is not one of them