They list those little harbor freight style ceramic heaters as 150 watts which plug into the cigarette lighter socket, the elements are 300w (possibly less, at 6v they only draw 75w) Secondarily the batteries put out a large number of amps (mine says 850 cold cranking amps):

watts/volts=amps
300w/12v= 25amps

Correct me if I'm wrong (which is probable, my vehicle electrical background is limited)