As noted by Reddit and The 8-Bit, as of software update 14.3 the HomePod mini can now be powered via 18W adapters.
This means that users will now be able to use the 18W adapter that shipped with the iPhone 11 to power their HomePod minis. Prior to the 14.3 update, users who tried power the wireless speakers with an 18W adapter were met by an orange glowing light on the HomePod, which according to Apple, means that the HomePod mini is plugged into a power adapter that’s not rated 20W (9V ~2.22A).
However, the HomePod mini will only accept power from 18W chargers with a specific power profile. As noted by The 8-Bit, there’s no official word on what the power profile exactly is, “which means you might have to struggle to find the 18W adapter that works.”