The equation for power is basically P = I x V, where I is amps and V volts. Most car systems are 12V. So 12W is 1 amp at 12 volts. So a 5 amp fuse would carry 60 W, which is fine for a 12+12 charger.
Many accessory plugs may have much higher fuse ratings as they have to power things that take more than 60 W. I think many will have a 15 amp fuse to power things like a cig lighter or an air compressor for tires.
Fixed that, (5 X 12 = 60)
😉
Not only that, when a car is running the voltage is higher than 12 volts, almost always 13.6 Volts so the output is even higher (78 Watts).
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It seems to me that Apple does not want manufacturers to sell USB chargers with detachable lightning cables included, it seems like only Apple will be able to sell USB to lightning cables.