You can calculate the current draw of any device by taking the power and divide it by the voltage. For a simple calculation and to build in a safety factor (ie when determining a value for a fuse) in the car environment, divide the power usage by ten to come up with the current draw. ie. if you are running 2 low beams at 55w each or 110 watts total, you would have 11A of current draw for running low beam headlights.

To be exact and accurate, divide 110W by the actual measured voltage that the car system sees. Therefore, if operating voltage is 13.5V rather than the std 12V, you could determine a more real current draw.

110w/13.5v

Cheers,

Tim