18w, because when the xs was released, the maximum charging power supported by the mobile phone did not exceed 20w, in fact, everyone understands the charging of Apple, after all, it is a 5w charger that has been used for about 10 years, so even if it is a higher-power charger, the Apple phone can not hit such a high charging power. Then why waste a few dollars for a more expensive high-power charger, the right one is the best.
18 watts fast charge, the iPhone xs only supports 18 watts fast charge.