It depends on the alternator (if it's capable of charging for the system) and what the regulator is set at. Generally, with most cars with stock alternators, the voltage is between 13-14 volts. It depends on engine rpm as well, you'll always have more current capability at cruising speed than you will at idle. If your dash has a guage instead of a dummy light, that will show you how many volts your system is getting. The 12.5, 13.8, and 14.4 are just general measuring parameters, I always go by 12.5 if possible, you're assured the power that way. It really doesn't make a huge difference, though. If you get a custom alternator, you can just get the builder to set the voltage regulator to 14.4V and it will ensure that you get that voltage.
yup 12.5 volts is the battery voltage (car engine off) and 13.8 to 14.4 volts is the alternator charging voltage above about 900RPM. remember the alternator has to create a higher voltage tahn the battery in order to generate a forward bias to charge the battery. If the alternator put out the same voltage as the battery, the battery wouldn't get charged to full capacity while the motor is running.
