Start with this post from Joe Rossignol, MacRumors, titled HomePod Mini Now Works With Select 18W Chargers Following 14.3 Software Update.
Apple includes a 20W power adapter with the HomePod mini, but many customers may have an 18W power adapter from an iPhone 11 Pro or other device.
Point being, if you have an 18W charger and buy a HomePod mini, use the 18W for your HomePod, now you have a 20W for general use.
Moving on to the headline linked post from John Gruber:
The only way to tell Apple’s new 20W charger apart from their old 18W charger is to look at the hard-to-read small print (light gray text on a white background, a veritable crime against accessibility). And even when you read the small print, you have to know that Apple’s 20W chargers say “20W” on them and their 18W chargers aren’t labeled with a wattage. Seriously, Apple’s 18W charger doesn’t say “18W” — the only way to know it’s an 18W charger is to examine the even-harder-to-read smallest-of-small print and know that it’s stated maximum output of “9V × 2A” is 18W. (Their 20W charger is 9V × 2.2A, so it’s really a 19.8W charger.)
First, check that math at the end. Good to know the W=V*A equation.
That aside, amazing that it is so hard to tell the difference between a 20W and 18W charger. Read this, then go look at your chargers, maybe make some labels so you can easily tell which is which.
Moving on:
So on the one hand, because the HomePod Mini includes the 20W charger, it was fine that it didn’t work with the old 18W charger. But on the other hand, if you ever toss the 20W charger into a bag or drawer along with an Apple 18W charger, you needed an extraordinary amount of knowledge to be able to know which charger the HomePod Mini required. Not sure how much work Apple had to put into the 14.3 software update to make the HomePod Mini work with the 18W charger too, but I’m glad they did. It’s too confusing otherwise.
Lots of details here, but update to the latest HomePod OS and you’ll have the ability to swap out the 18W and 20W chargers.
Gruber’s post is worth reading, especially with details on what he discovered about his Magic Keyboard and Apple’s slightly older 29W USB-C power adapter, which looks exactly like Apple’s more recent 30W USB-C power adapter.