MarkT:
Most linear regulators are very current efficient(*), but power efficiency depends on the ratio
of output voltage to input voltage.(*) they draw something in the order of 1mA to power themselves, so a 500mA regulator providing 500mA
will be drawing 501mA or so, but wasting 3.5W due to the 7V difference between input and output.
polymorph:
Read the datasheet.http://cds.linear.com/docs/en/datasheet/8021fd.pdf
It is a switchmode power supply. It does -not- require 12V in to get 5V out. However, with 12V in and 5V out (page 5 of the datasheet, top center), with 500mA out, it requires 250mA in.
I appologize for my imprecision in specifics, I didn't bother to look at the data sheet, nor does it really affect my point. The capacity/specifications of the OP's car battery will never cause him/her to have any issues with his projects power supply-If it can start a car it can provide whatever current a 0.5 A power supply is capable of demanding.