Power rating for 9VAC source

Hello, I want to install the iotawatt inside the power panel where space is scarce.

I will use a MW HDR15-5 (2.4A 5V) din-rail psu, but I have no idea what are the power needs of iotawatt for the 9vac ref or the voltage tolerance.
Is this just used for frequency reference? As I can’t find any din-rail 9vAC source, can I use something like this: https://grobotronics.com/transformer-230v-9v-10va.html ?
Alternatively I’ve found din-rail “bell” transformers (ABB TM10/12 or Legrand 413091) that outputs 8,4,12 VAC on the secondary, can I use the 8V reliably?
Thanks for your input.

Be aware that the IoTaWatt WiFi may not work when mounted inside a steel box.

The AC reference can be any voltage from about 6V to 15V true rms. That said, you will need to calibrate a “generic” model and the phase-shift will be unknown, which can introduce slight errors with ordinary circuits.

The AC adapter is only a reference and will draw less than 1mA, so no worries there.

Thank you for this info, 12VAC is easy to find.