Ohms law comes into play here........
Volts-Watts-Amps-Ohms. With any two of those numbers you can come up with the others.
In this case you have 12 Volts x .5 Ohms (typical for a good plug) = 24 amps just as D-S-D said.
Using that law you can see how a bad set of plugs can really kill your batteries in a hurry.