The following graph shows the electrical current the alternator can deliver at various RPM. The red area is the maximum current needed to run the ECU and a single ignition system. Adding the red, brown and green area gives the total current needed to operate the engine with a double ignition. The yellow area is the net current available from the alternator to operate other electrical aircraft systems and charge the battery.
30 Amp alternator, standard on 4 cyl engines
50Amp alternator, standard on 6 cyl engines ( can be installed on 4 cyl but this is optional)