I'm looking around for DC or USB powered thermometers but I can't seem to find any. Granted, the battery powered ones last many years, but I'm looking to keep my options open. Zigbee devices are preferable. Anyone know?

ESPHome + DHT22 or DSB1820 + ESP32-C6 and a small hobby box plus power supply (old USB phone charger) and you can build your own.

https://esphome.io/components/zigbee/

Like actual thermometer or temperature sensor?

I have so sooo many temperature sensors in my network. Thermostats ($15) plant moisture sensors ($3-$6 from overseas but battery) my window AC unit apparently has several sensors home assistant picked up, a cheap air quality monitor, temp/relay units, and a smart humidifier.

Seems a waste to build dedicated hardware when cheap options just bundle temp sensors willy-nilly.

I'm not sure why you'd NOT want a battery powered one, because as you say, the batteries last years, and a USB powered one would waste much more electricity over time, but I digress.

If you have any mini SoC boards or ESP32 devices laying around, you can get dirt cheap 2-pin external temp sensors. Works on anything with a GPIO pin out that supports simple I/O.

Anything you find that is Zigbee by nature will be battery powered, however, you might be able to find a device with a different purpose that happens to have temp sensors that export data you could monitor.

Discontinued IKEA VINDSTYRKA (Zigbee) or ALPSTUGa (Matter) come to mind.

Most of mine are, however plug-in devices also work as repeaters such as my air quality monitor (USB) and smart plugs. I've found they really help with dead zones.

I might do this option, thanks!

I do have a Pico sitting around. Maybe I can use it for a thermometer project.

Picos are very bad for battery applications. If you do end up doing this, make sure it is mains-powered like your original thought.

Esp32s, MSP430s, and nRF chips have sleep currents down to around 5uA or less.

Picos have sleep currents of 1-4mA. That is 1000x higher and the battery will only last for a few days instead of a few years.

I recognize it's not the answer to your question, but the ZigBee ones I have run on AAA batteries and last about a year on a charge.

With Eneloops, even after a decade they still keep over half their charge capacity. Could probably go 15-20 years per battery in regular trickled use with at most quarterly swaps - I think that's kinda ideal operation for these guys.

One battery every 15 years, and an annual swap when it drops off my HA is a small price to pay for the flexibility of being able to deploy anywhere with zero infrastructure. I have a dozen sensors and would not want to have to wire each one.

FYI - Stock white charger that came with eneloops did not like how low the voltage gets when they run low in the temperature sensors, so I swapped for the "super pro" charger and it's significantly better at managing the batteries: https://camelcamelcamel.com/product/B0B2WXJH43

I modded all of my sensors by adding a usb-c port and 5v-3.3v mini converters

Perk is, if done right you can still use battery if needed

Thanks for the insight, good to know.

I literally just did this over the past few weeks. I bought four ESP32-C3 minis on expansion boards (The expansion boards come with PH2.0mm connectors and built in battery charging management, so that I can add battery power next - already ordered the LiPo batteries. At the moment they're plugged into USB-C phone chargers) and four SHT-41 temperature and humidity meters. I then installed the espressif development environment and (don't shoot me, I was under time pressure) vibe-coded the setup for the ESP boards: they broadcast the readings every 10 seconds via mosquitto.

I also bought a barebones raspberry pi and made it subscribe to the mosquitto feeds and write the data to a sqlite database, and created a basic flask website that reads the data from sqlite and shows the graphs and the temperature readings to anyone on my local wifi.

My code is on codeberg - I'd be happy to share the link if any of this is relevant to you.