Many factors affect the RMS wattage rating. Such as the size of the subwoofer, the number of speakers in your car and the type of amplifier. Generally, most car audio subwoofers range from 200watts to 1200 watts on average, but if you have a powerful amplifier, then there is no need to be that much. For example, if you have a 600 watt amplifier, buying a 300 watts subwoofer should be enough for your car..