Can I damage an iPhone if I use a 24 watt (5volt) charger?
The 6/6S can take advantage of up to 1.6 amps if the USB adapter can supply it. Using a higher capacity adapter will not hurt the phone, but won't help either. An iPad adapter is 2.2 amps, the iPhone "cube" is 1 amp. So an iPad adapter will charge an iPhone 6 or 6S series slightly faster. Not true for the 5 and earlier, which will only use 1 amp regardless of what is available.