Yes, the Anker 20w Fast Charge can charge the iPhone XS. The iPhone XS supports fast charging, with a maximum charging power of 18W, while the Anker 20w fast charging power is 20W, so it can provide enough power to quickly charge the iPhone XS. Using Anker 20w Fast charger can greatly reduce charging time and improve charging efficiency, so that you can enjoy the use of iPhone XS faster.
You can use 20w fast charge, because it can quickly fill the rechargeable battery, which will be more convenient