In the past, 18W was common for sub 20W charging using USB Power Delivery technology. With the release of the Apple 20W Charger, which pairs well with the Apple MagSafe Charger and iPhone 12, don't be surprised if 20W (9V@2.22A) becomes more commonly supported.
Note if you need a smaller 20W charger, the Anker PowerPort PD Nano 20W Charger can be used but note this charger doesn't support the special Apple vendor identification that is used by the Apple 20W Charger.