Well, you seem very knowledgeable on the subject and I appreciate your effort to help me understand and ask the right questions.
Why would the value I can measure with the other energy meter be the correct one? Indeed, good question. I did not question it.
It's an energy meter that I bought many years ago in the market that has a display and different functions and plugs in between the wall outlet and your device. And so, this device displays 14W (approximately).
On the Shelly side I always have these 2 instantaneous voltage/current values displayed (231V/60mA). Is it a coincidence that I apply U*I and also find around 14W?
Should I deduce from this that my off-the-shelf energy meter does the simplistic calculation of U*I without taking PF or any other parameter into account? I don't have it on hand but it could also be that I simply focused on displaying the wrong function. I'll look in that direction.
In addition, this would be excellent news, it would mean that the Shelly may be measuring just right and that my NAS would only consume 2 W in shutdown mode. And that's more than acceptable.
Thank you for the link. Good work! Very interesting. It confirms that you know very well the subject ;-). What I retain is the risk of overheating and therefore of premature aging of the capacitors if we are close to 10A. Nobody is perfect...
I have a lot to read and to learn about these Shelly devices. Again thank you.