false instantaneous power value while Voltage and Current are correct

    Hi, I am testing the Shelly Plus 2PM and its energy meter function. For this test, I decided to control the power supply of my Synology NAS in order to reduce the consumption to almost 0W. For this test I use the Shelly's switch_0.

    The power consumption of my NAS is about 45-50W in normal operation and still consumes 14W in shutdown state.
    By switching the Shelly switch_0 to OFF, the consumption of the NAS is almost 0W (there remains the consumption of the shelly... ). So nothing to say about it. It works.

    Where I find energy meter reading error is in case the Shelly switch_0 is ON and my NAS is down. I'm supposed to read 14W while the Shelly shows me 2W. What is strange is that the Shelly also shows me the instantaneous voltage (234.6V) and current (0.059A). Applying the formula P=UI, I find: 234.6*0.059=13.84W. Which corresponds to the 14W displayed by another measuring instrument and not the 2W.
    On the other hand, when I turn on my NAS, the instantaneous power value measured by the Shelly is identical to the value measured by the measuring instrument (45-50W). In the latter case, the Shelly also displays the power factor.
    So, I don't know how the instantaneous power is calculated in the Shelly. But if it is a simple U*I, the value should be correct in any cases.
    Any help to understand where I'm wrong is welcome.
    Laurent

    Welcome to the forum!

    To your issue: Power in systems with alternating current isn’t P = U * I, but P = U * I * cos φ. The third factor is called “Power Factor”, which may be roghly described as the angle between current and voltage. Hence, this is the reason for the supposed misreading. Especially electronic power supplies exhibit large deviation between current and voltage…

    „Habt Geduld. Alle Dinge sind schwierig, bevor sie einfach werden!“ (aus Frankreich)

    „Nothing in life is to be feared, it is only to be understood.“ (Marie Curie, 1867-1934)

    „Es reicht nicht“, rief Schiller, „Gedankenfreiheit zu fordern, man muß auch denken können, sonst fordert man Gedankenlosigkeitsfreiheit und die ist die Freiheit zur Dummheit, welche wiederum die schlimmste Unfreiheit überhaupt ist!“
    (Aus „Besuch aus Weimar“ von Gert Heidenreich, Schriftsteller, *1944 in Eberswalde)

    Thank you for this important comment. Indeed I must take into account the PF in the calculation. Moreover, it is part of the 3 information displayed by the Shelly (V, I, PF) when my NAS is ON.

    After some research, I saw that one of the chips used for energy measurement is an ADE7953. It is apparently this chip that is used in the Shelly 2.5. Going through the datasheet, I can only bow to those who wrote the software part that manages this chip. There are so many settings and modes. You have to know your subject well. In short, I digress.

    So I still don't understand why the instantaneous power (false value) is displayed and not the PF value. Is this a deliberate choice of the developers?

    For example, if it is the ADE7953 which is also used in the Shelly Plus 2PM, to see the number of programmable parameters, I am surprised that it is not possible to correctly measure the phase shift between the voltage and the current in in case my NAS is OFF (14W, 321V/60mA). Is there a threshold that has been determined by the devs below which they don't show the PF? So they shouldn't display the wrong instantaneous power value either (2W is displayed in my case).

    Far be it from me to criticize the choices of the developers, I just want to understand...

    Thanks for your thoughts, lhac5vet ! It would be very interesting and instructive to have a deep insight in the head of a developer - but it is not (yet?) possible. I’m convinced, the ADE7953 would be able to indicate the active power of a idle running power supply, although it is far from being an easy job: Cause there is no well defined phase angle between current and voltage! Current is flowing for a short moment only, at the maximum of the voltage curve, to maintain the charge of the capacitor(s) in the DC HV circuit. It is merely a calculation with harmonics, which have to be determined by some FFT operations in the Energy Meter chip…

    But what leads you to declare the 2 W indication of the Shelly to be faulty? And with which apparatus did you measure the 14 W?

    At last, a kind of a bonus: Here you’ll find an insight to the Shelly plus 2PM - unfortunately in German language:

    Thema

    Shelly plus 2PM: teardown, Stromaufnahme

    Wie sieht es im Innern des Shelly plus 2PM aus? Wieviel Leistung verbraucht er? Das ist das Thema dieses Threads…

    smarthome-forum.eu/attachment/35102/

    Allterco ist es gelungen, den äußerst dichten Aufbau des Shelly 2 .5 noch kompakter zu gestalten. Die frei gewordene Leiterplatten-Fläche wurde dazu genutzt, die WLAN -Antenne als Leiterbahn auszuführen. So konnte die problematische Einklebe-Antenne des Shelly 2 .5 entfallen!

    smarthome-forum.eu/attachment/35103/

    Entfernt man die Klemmleiste,…
    thgoebel
    11. Oktober 2022 um 13:39

    „Habt Geduld. Alle Dinge sind schwierig, bevor sie einfach werden!“ (aus Frankreich)

    „Nothing in life is to be feared, it is only to be understood.“ (Marie Curie, 1867-1934)

    „Es reicht nicht“, rief Schiller, „Gedankenfreiheit zu fordern, man muß auch denken können, sonst fordert man Gedankenlosigkeitsfreiheit und die ist die Freiheit zur Dummheit, welche wiederum die schlimmste Unfreiheit überhaupt ist!“
    (Aus „Besuch aus Weimar“ von Gert Heidenreich, Schriftsteller, *1944 in Eberswalde)

    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.

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