All Apple chargers are 5 volts. Amps is the amount of current flowing into your device to charge your battery. Watts are easily calculated V x A = W
- iPhone charger is 5 volts and is 1 Amp so it has 5 watts
- iPad charger is 5 volts and is 2.1 Amp so it is 10 watts
Now you see the problem? Will it work Yes, but you are forcing to much current to your battery. End result is that you are decreasing the life time of your iPhone battery.
