The 5W CAN-bus is used for interior applications, and license plate lighting. The 21W CANbus adapter is used for exterior applications such as position light, low/high beam lights. Always refers to the original wattage of the halogen/conventional lamp. As an example, a P21W usually drawn 21W, when our LED-RED [≈P21W] has a wattage of 1.9W. The gap is then 21W-1.9W = 19.1W. This means that to compensate this gap of wattage, you should use a Philips 21W CANbus.