This is an interesting question. Theoretically, the maximum output from a 12v "cigarette lighter" socket is 180 watts, since most run through a 15 amp fuse (15Ax12V=180W). In practice, of course, it's far lower. The last thing any manufacturer wants is for you to blow fuses in your car trying to recharge three phones, a laptop and a curling iron or something. For the most part, the fastest car chargers will charge your phone in a similar, though probably somewhat slower, timeframe as a decent charger at home.