A reason for the PI model is that you can order the Pi in Europe so you don’t pay import taxes on this part.
The OS is a cheaper that way and has import taxes since it comes from the US.
I power the OS using the trafo of my doorbell (currently without issues)
@wokkeltje Do you have an overview of attached devices? I mean what kind of “waterpump / -controller” do you use, other things…?
Where have you bought your device?