Current Determination - 12V to 5V (USB Output)
The 12 V input has to supply at least 15 W.  Actually it will always be a bit more due to the inefficiency of the converter.

For example, let's say your converter is 90% efficient.  That means to produce 15 W out, it needs (15 W)/90% = 16.7 W in.  For 12 V input, that means it will draw (16.7 W)/(12 V) = 1.4 A.

Note that car power can vary considerably.  The battery may be around 12 V when everything else is off, but is usually held at around 13.6 V when the engine is running.  It can dip to 9 V or less during starting, and there can be 10s of volts of spikes occasionally during operation.