Electricity and power:

Watts = Amps x Volts.

The basic charging voltage is 13.5-14v, so a "12v" system is actually a 13-14v system.

The stock alternator produces a max of 50-55A at 14v, or 700 to 770W. This is, btw, about 1HP (1HP = 746W). Alternators turn physical motion into electricity with about 50% efficiency, so the alternator uses about 2HP from the engine to generate its max power. The alternator only generates as much power as needed, so it doesn't use 2HP all the time.

Some common loads:

Ignition 3A
Injection 3A
Fuel Pump 4A
Head/tail/dash lights 10A
Brake lights 5A
Turn signals 4A (each side, so 8A for hazards)
Blower fans 20A (both)
Stereo 2-5A
Wipers 8A

So, with everything on, that's 44A, about 616W, or about 1.5HP taken from the engine to drive everything. With just the ignition/injection/fuel pump on, it's only 0.4HP. If you're running carbs, the fuel pump is likely only 2A (less pressure = less work), so it's more like 75W total, or 0.1HP.