I am replacing the headlights in my old lawn tractor, and wondering what size resistor to use. The new lights are 42w total, and draw 1.5A at 12V.

 

Usually LED headlights have a built-in current-limiting circuit.
If each headlight draws 1.5A at 12V then it is 18W. Two headlights are 36W, not 42W

Such a high power uses many single LEDs maybe inside one larger LED. They probably use three 3V LEDs in series (then columns in parallel) then use a current-limiter for the remaining 3V.

 

Just plug them in. If they are for 12V, they are ready to go without any mods.

 

Thanks for the help!