Figure out what voltage your car idles at, whether its 13.8 or 14.4 volts, and divide 3000/14.4 and that gives you your amperage. Watts = Volts x Amps... So supposing that your car idles at 14.4, 3000/14.4 = 208, So you would need a 200amp alternator to run your system... However, you could...