Power consumption of programmable switches


As a research group at IMDEA Networks Institute, Spain, we are looking for a device to measure the power consumption of our programmable Intel Tofino switches. Open Energy Monitor devices seem to be a strong alternative for metered-by-outlet PDUs. However, I would like to know the answers to my questions which I could not find after going through the docs before purchasing any.

  1. I would like to know which metered metrics emonTx4 (or EmonPi) can log in to Emoncms. Except for active power measurement results, which data (such as active current measurements (amps) of each current sensor, etc.) we can monitor in Emoncms after metering.
    I am trying to understand what Open Energy Monitor devices can offer to monitor in terms of metered and calculated data and if they offer something different than metered-by-outlet PDUs.

  2. What is the metering accuracy of emonTx4 and emonPi?

  3. We are planning to use EmonTx4. I saw EmonTx4 with Wifi is in the market however it seems it is not available. It is not in stock now or it is not available for purchase yet?

Finally, which device do you suggest to meter power consumption in multiple switches separately? I think that EmonTx4 can meter power consumption of up to 6 devices. Is it correct?

Thanks a lot in advance for any help! Hoping that all the questions are clear enough!


Welcome, Bayza, to the OEM forum.

I can answer the technical questions, the availability of the emonTx4 with WiFi is for @gwil or The Shop to answer.

  1. By default, only the active power and voltage is sent by the emonTx. However, the rms current is available within the emonTx and only requires a small change to the software to send this to emonCMS. Unfortunately, no information about the reactive components of the current (and hence the power) is available.
  2. Without individual calibration of each channel, the accuracy of the emonTx4 should be within ±1.2% (Overview — OpenEnergyMonitor 0.0.1 documentation). When the “new” emonPi becomes available, this should be the same

Yes, but I expect a 6-channel expansion board to become available soon, taking the total up to 12 current/power channels. I have one, and the software for it is being tested.

I have not looked at this Emon32 - a modern energy monitoring implementation (I don’t understand Github) but it might be of interest.

Also worth detailed consideration: Emontx3-continuous - New continuous monitoring firmware but you would need to manufacture an equivalent device to the emonTx V3.

The emonTx4 with Wi-Fi option is back in stock: emonTx V4 with Onboard WiFi - Shop | OpenEnergyMonitor.

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Thanks a lot for the answers. It is very helpful.
I will check the devices you suggested.


@Gwilym Shouldn’t this really be called RF not wifi, according to the description it’s 433MHz RF not wifi?


This version has an on-board ESP8266 Wi-Fi module. This is in addition to the RFM69CW transceiver module, which is still there as it is on the standard offering.

Aha! Does that mean that the ESP8266 board is available as an add-on to the standard system?


Yes it does.

And I understand it will be inside the case, I’ve not seen the WiFi board, I think there will be plenty of room as it will sit next to the 6-channel expansion board. The p.c.b. has two lines of sockets (or provision for), one set labelled “UART_EXT” and the other labelled “SPI_EXT”.

Although the 433 MHz module is commonly referred to as RF, (to distinguish it from Wi-Fi)
the term RF means Radio Frequency, which applies to Wi-Fi as well.
(and anything else emanating an electromagnetic signal at frequencies below 300 GHz)

The 433 MHz radio should be referred to as a UHF radio/module.

Old habits die hard.

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They do indeed…

@Gwil - so when will it be available in the shop? Also, while the link you give above gets to the wifi version, if you go in through the shop, you only get to see the standard version. Shop probably needs a bit of maintenance?

Yes, correct. It sits neatly inside the aluminium case. On the standard emonTx4, these two sets of headers (“UART_EXT” and “SPI_EXT”) are not populated with female headers. So you would need to solder some female headers on to the board if you wanted to add the Wi-Fi board to an existing emonTx4.

It is currently listed under ‘Kits, Components & Development boards’. These are quite low volume; they are at present quite labour intensive to produce.

The Wi-Fi add-on boards are not available individually at present - I will check with Glyn and Trystan about this. As I said above, it requires soldering, so it is not going to be a suitable option for everyone. In the meantime, If you are interested, send me a PM or email the shop and we can arrange that.

That is a shame.

Thanks for the info - good to know - I’m hoping I can site things, so that I can use a serial connection but will give you a shout if things change.

Instead of soldering the headers, would an option be to use those tap in headers? Seem to remember seeing these for use with Pi Zeros.