I have the 30W charger. Someone did a test with all the different chargers & published the results, have a search for the thread. I wouldn't waste money if you have 18W as you're talking a few mins faster at most to charge 0-100%. That's not worth spending money on.
The 2018 iPad Pros can charge above 30W though, although not by much more, and sustains that charging speed for a fairly long time.
Third Party high power USB-C Chargers are relatively cheap now, so it may be worth buying regardless. For example, this 60W Power Delivery charger is only $25 and has a few legacy USB-A ports if you need them.
[doublepost=1551457495][/doublepost]This 45W one is more minimalistic for only $14.
This setup, vs the factory sent 18W version , if much faster going from 10% up to full. I noticed right away.
The cable has to also support the fast charging current. On my prior 2nd gen ipad i had a 30W Anker and an old model if certified cable and i also got faster speeds over the factory supplied apple charger.
I use a usb hub with Ethernet daily for several hours and the hub plus Ethernet will drain the battery super quick. With this setup the higher current will keep the hub and Ethernet powered and also keep the tablet happy by supplying more current so that it charges the battery, rather than “maintain” the battery (not supplying enough current to actually recharge)—if that makes sense.