How do you measure geyser temp?
I installed a new solar geyser recently, and got with it a super cheap temperature sensor and display (I declined the GeyserWise, knowing I wanted to do something myself).
Since I knew the installer well, I convinced him to lend me another probe and did some tests until I eventually figured out it was a NTC thermistor. Then I used a Wemos D1 I had, built a basic voltage divider on some breadboard, loaded ESPHome onto it and configured it into a
basic digital temperature sensor.
I used the second probe/sensor to calibrate it (using another temperature sensor I had, and a bunch of different temperatures such as boiling water, warm water, room temp and the fridge), then moved the end of the already-installed probe from where it was in my kitchen to my study, and plugged it into the Wemos.
The ESPHome firmware/add-on automatically expose it as a sensor in Home Assistant for me.

(I need to mount it! Theres also a second sensor to measure temperature and humidity in my study).
It's useful. I control the geyser element from a Sonoff Basic + contactor, so I can, depending on the PV yield and time of day and geyser temperature, switch on the element for a couple of minutes and let it switch off when reaching target temperature, or consumed X kWh of energy or ran for Y minutes. Basically to "boost" the temperature when the solar geyser by itself can't get to where we want it. Or as you can see, run it for a little while to get the water back to 50 degrees for the significant other's shower (just before 0:00 last night).
The Aqara gateway shows pure red when 50 or higher, or pure blue when 30 or lower, and then some shade of pink/purple somewhere in between 30 and 50. When it goes from red to pink, the SO immediately goes and shower!
(This is for the 43 degrees above).