The cheapest M2 Mini is $600. That's generally overkill for personal servers. They also have an unknown reliability record, and the older Mac Minis had a tendency to eat storage devices by limiting "fan noise" until temperatures were at the upper threshold of the spec. In the new ones the storage is permanently attached, which is... worrying.

One of the better options if you don't need a lot of internal storage is old laptops. They're cheap, low power, have a built-in monitor and keyboard and you don't need a separate UPS (who cares if the internal battery "only" lasts an hour).

The Mac Mini is pretty tempting, but I wanted something even lower-power. 15W is possible for Celeron mini pcs. They are around. I ended up getting a Quieter3Q which is a Celeron-based 15W, fanless and cheaper than a Mac Mini.