In short - you can use it with iPhone 11 and it will draw a bit more power when charging over cable (it can draw 22-23W max), but I doubt you will see any big difference in charging time when compared to the 18W charger (maybe a few min). When charging wirelessly - you will see no difference - iPhone 11 can charge max with 7.5W and does not support the new MagSafe protocol (which is 15W anyway).