For a general idea of how much a current an alternator typically needed to produce while driving, here's some I current draws I recently measured on a Jeep wagoneer with side markers. Solid state Ignition and field, 3.5 amps Parking, side markers, and instrument panel lights, 5 amps Headlights, 9 amps Heater fan on maximum, 9.8 amps Voltage during measurements was 14.3 Volts