Why The prefix milli means one thousandth, so to convert milliwatts to watts you divide by 1000. 500 / 1000 = 0.5 watt. Going the other way, multiply watts by 1000 to get milliwatts.
Watch out The choices in the hundreds of thousands or millions come from multiplying by 1000 instead of dividing; that would be the conversion for kilowatts or megawatts, which are larger units, not smaller ones.
Milli = thousandth. Small prefix means a smaller number of watts, so divide by 1000 and the decimal moves left.