P21W also belongs to the family Group 2 automotive bulbs. You can use them for anything except headlights. So when you want to give your signaling or brake lights the upgrade that they deserve, you know what to buy.
These are dual beam lamps. If you look at one, you will realize that it has two filaments. As such, a single bulb can produce both the low and high beam lights without problems. Additionally, P21W uses BA15s base and have a favorable nominal power of 21 watts at 6 volts. These unique attributes make bulbs suitable for all the legalized functions.
You will find plenty of halogen P21W bulbs on the market. In most cases, they are adequate to meet the legal functions of the lamp. But if you want to use bulbs on the exterior of your car, you may need to upgrade them to get a brighter light for an extended time.
P21Ws last for a few months. They have a lifespan of about 500 hours. If you upgrade them to HID, LED, or halogen with a higher lumen, you can be sure you will get the service for close to 5,000 hours. Your improved bulbs will also give you a brighter light.