Unless it a very high powered LED, to get 5Watts of heat generation in a LED's series resistor would be unusual when powered from a 12V source.
Standard LED's are rated around 10mA to 20mA, look at this chart for typical voltage drops.
So say you had a White LED with Vfwd of 3.2V to get a 20mA current thru the LED from 12V would be [12v-3.2v]/0.02 =440R
The wattage/heat in the resistor would only be 0.176W
So to get heating of 5Watts of a resistor would mean [using a White LED] would be [8.8V^2] /5 =15.48R
So the current would be I = 8.8/15.48 = 0.55Amps