OpenEnergyMonitor Community

Monitoring: CT sensors, modbus meters and/or plugs?

I still want to do energy monitoring, but there are plenty of options. So it’s hard to make up my mind. I have read some topics (like ‘New build house: improving on clip on Cts’), but I still have some questions. Eg. what is the best to monitor? What I think is an overview:

  • CT sensors. Doesn’t do much in itself: you need something like OEM or build your own.
  • Meters with modbus communication, preferably DIN rail mounted. Eg. SDM120 or PZEM-016. Disadvantage: you need space in the consumer unit. Two versions: CT or in-line. I don’t know how the last one works inside. Is it CT based as well?
  • Pulse counting. Is like counting mile markers as you go along a motorway rather than reading your odometer, very accurate if you don’t miss any, but hard to ensure you definitely always count them all (thank you @pb66 for the visual explanation). If possible, better to avoid.
  • Plugs, where you can easily monitor a single device (eg. microwave, electric heater, …). Examples are those from Sonoff or Shelly.

Advantages/disadvantages. Is one more accurate than the other? Or more stable? A try:

  • CT sensors: non invasive, so safer to operate, …?
  • In-line: invasive, you have to rewire. So less safe to operate. …?
  • Plugs: non invasive, but only interesting if you can really plug something. So not for overall electricty consumption or PV production.

To have good energy monitoring, you’ll have to bring everything together, eg. into Emoncms.

  • Looking at the learn-part of OEM: it doesn’t mention modbus.
  • The guide-part focuses on CT, pulse counting and temperature nodes. Modbus is briefly mentioned in ‘EmonHub Interfacers’. It doesn’t go into details, e.g. when you have different modbus meters and what to do when you want to daisy chain them.
  • The guide-part talks about ‘Sonoff WiFi Smart Plug Scheduling’ as part of EV Charging & Control (although I don’t think you can use Sonoff for big load of an EV). But that is about sheduling, not about energy monitoring.

About bringing everything together: I guess time management is key. If your plug logs 10 seconds later then your CT or modbus, one can jump to the wrong conclusions. The same for eg. 2x EmonTx. Or is time management not really an issue?

As for my conclusion: if I read the webpages on ‘Learn’ or ‘Guide’, I guess the best way for one to start with energy monitoring is to use CT with EmonTx/EmonPi/… and Emoncms. Correct? Be able to integrate plugs into Emoncms seems nice to me, but it’s not something for my near future :-).

It’s on my radar, but unfortunately, nobody seems interested in documenting things like that. It’s been decades since I had anything to do with Modbus, so although I’m trying to update ‘Learn’ as and when I can, Modbus is something that someone else must make the major contribution to.

Good documentation is key, but unfortunately not very popular to write. I guess the table of contents also needs an update, as ‘gas metering’ or ‘temperature’ doesn’t really fit the ‘Electricity Monitoring’ heading. Maybe ‘inputs’ is a better heading? I also guess that users have more experience with wiki’s (eg. than with documentation via Github. Maybe a heading ‘Contribute’ can help?

But I’m drifting away from the topic. Getting back to ‘CT or kWh meters with modbus’ and after reading a bit more, I would guess that an kWh meter for a 3 phase setup is the better setup (namely more accurate). I jump to that conclusion by reading the info on the shop.

The emonTx with the default firmware is suitable for use on a single-phase supply, or on a split-phase supply, or on one phase of a multi-phase supply. Firmware is available which will allow for ‘approximate’ 3-phase monitoring (described here), which we can upload prior to shipping if it is requested.

Eg. the Eastron SDM72D or SDM72D-M which is bidirectional and can show both imported and exported kWh. I don’t know if there are difference between SDM72D and SDM72D-M, but I only seem to be able to find SDM72D in shops, while finding only the manual of SDM72D-M. Maybe the ‘-M’ only stands for Modbus or MID-certified?



  • SDM72DR - 100A Input fixed Energy(kWh) and resettable Energy(kWh), Fixed pulsed output
  • SDM72BI – 100A Input Import and export Energy, Fixed pulsed output
  • SDM72M - 100A Input Modbus RS485 (RTU) & fixed Pulsed Output

It doesn’t mention SDM72D-M - but the image is exactly that. If you look at the datasheet, that specifies that the -M has both Pulse and RS485 Modbus RTU output, and only the -M description mentions Modbus.

With that, you have guaranteed accuracy, which of course you can never have when all three phase voltages cannot be measured.