Can I use a 20W charger for iPhone 7?

iPhone 7 only supports fast charging up to 12 watts, so using a 20-watt charger will not charge your device any faster than using a 12-watt charger.

sure

Watt  is like how fast your car can drive, but the speed limit is how fast your car do drive

 the iphone draw as much current as it was build to use when charging, the fact that

a charger can provide a lot more just means it's not having its full capacitive used

Response to Foisal_ahmed Charging iPhone 7

I have an iPhone 7 as well , that I attempted to charge with my iPad 18 watt charger , it didn't work out .

Here is more info :

About Apple USB Power Adapters

https://support.apple.com/en-us/HT210133?