Q: Iphone 6s plus car charger ( amper/ W)

One example: http://www.apple.com/shop/product/HJ3J2ZM/A/belkin-car-charger-24a-12w-with-buil t-in-lightning-cable?fnode=97

5V/12W 2.4A

5volts amps are of no relevance as long as there are enough

you can charge it with a 20000Watt charger as long as it's 5volts

the device will draw as many amps as its charging system was build to draw

amps * volts = watt