New Install - Monitoring WITHOUT CT clamps


New here, so please be gentle :).

I’m pre-planning for a PV + Battery install, including and Islanding implementation (Critical/Normal circuits).

As part of the install (UK), there’ll obviously be a few smart meters being installed as part of the installation.

As I’m needing to replace my existing (small/old/a mess) CU for the circuit “separation”, I thought “why not include additional smart meters at strategic points in across the circuit to assist with local comprehensive monitoring that is “outside” of the inverter eco-system”

I see that the OpenEnergy products cater for CTs, but are there hardwired comms options available? I was wondering if the OpenEnergy eco system can leverage in-line smart meters rather than CT clamps (I’m not too keen on clamps and fly-leads flapping all over the place).

And as I’m needing to size a replacement CU, I’m also keen to understand how I might monitor individual circuits (Hob, Oven, Upstairs lights, etc.), again with strategically placed in-line monitors, that can feed data to OpenEnergy?

Thank you in advance for any guidance.


Hello @rashdown_online one option that we have been using for heat pump monitoring is to use SDM120 modbus meters, reading from these using the emonHub modbus interfacer running on an raspberry Pi, there’s a bit more about the configuration for this here: emonHub Interfacers — OpenEnergyMonitor 0.0.1 documentation

and discussion of this approach for heat pump monitoring: Heat Pump Monitoring — OpenEnergyMonitor 0.0.1 documentation

we sell these in the shop SDM120 Modbus MID 45A - Shop | OpenEnergyMonitor

Thanks @TrystanLea - That’s really helpful. And it’s got me thinking about how best to deploy those little monitors! I’m now thinking to deploy at each circuit in my sole CU. Are there input limitations on the number of SDM120 meters that can physically connect via USB (can I use USB hubs?)?

Can I ask, is there an OpenEnergy “Integration” with Fronius hybrid inverters? If so, then I’d not require additional meters through out the outer circuits - just focus on installing at discrete circuit level.

Thanks in advance for your help.


Hi Roland. I too am looking to install a similar system, specifically choosing Fronius for its good support of local monitoring APIs, and the more I read about Fronius the better I like them. I like your idea of adding extra current monitors on the various circuits.

I don’t know any USB input limits that may be specific to EmonCMS, but my general knowledge of USB tells me connecting more of them through a hub would certainly be supported at the USB level as that’s a core part of how the USB standard works.

Have you decided whether to use OpenEnergy/EmonCMS alone as your central monitoring system? The alternative is to attach it to Home Assistant (or another) general purpose home automation. I’m looking at the latter: I have installed HA (inside YunoHost, for the time being). Then will use the HA-Fronius integration to monitor the Fronius inverter, outside of EmonCMS. And if I need EmonCMS as well for monitoring additional inputs, I would use the HA-EmonCMS integration .

Hi @julianfoad . That’s what I was hoping/thinking too for the USB. We can see…

I’m also very taken with the Fronius GEN24 PLUS and OHMPilot solution, plus a BYD HVS7.7 battery. Not so taken with premium price point though! I’ve got some real thinking to do over the holidays on which option to take. The ROI numbers over the long term, no matter which solution is taken, are much more attractive these days, and will be even more so when my current cap of £0.28/unit ends!!!

I’m not too sure on having both HA, Emoncms, or both, tbh. I have an old v3 Pi as a test bed for HA. I don’t do automations (or not yet). But I’ve got it linked up to my IHD (Hildebrand) and it’s working fine. And I’ve seen the Fronius integration, yes. Again, lots to think about.

And as for the in-line meters in strategic places, I’m thinking it can’t hurt as I’m switching out my CU anyway. These, in addition to some Shelly relays at the lights (for now) might complement each other quite well.

But who knows… Got to get the PV+battery first.


Hi @TrystanLea .

I have another question, if that’s OK.

Should I include for a 100A smart meter at the grid-point, too (SDM230 – 100A Input Modbus RS485 (RTU) & Pulsed Outputs)? Or would it be better for the EmonCMS to connect to my hybrid inverter only?

I ask, as this is a fresh install for all electrics at my Distribution point.

Thanks in advance


Just a note about the SDM120 meters.
They use modbus which is a RS485 connection. These daisy chain on their own, each meter will need its own ID. You’ll need a RS485 to USB converter (only 1).
then you can read them. I think there’s a limit of 32 meters - per usb adapter.
Each will use a din slot in your CU though - so it will take up space you would want for RCBO’s

@David Thank you very much for the post! Every day is a skool day!

I’m replacing the CU anyway so might as well size it to accommodate the meters.

And as I’ll only need 1 USB converter, that’ll save a few bob!!!

Thanks again.


This is the one I’m using

as you can see they are nice and cheap.
You will also need a 150 ohm resistor and use that at the USB end. Its a termination resister, without it you’ll never read the meters.

@David That’s absolutely splendid! Thank you for the pointer.

So just to confirm: I daisy chain up to, say, 20 smart meters; get one of those USB/RS485 plug; terminate the cables to the terminal block with a 150Ohm resistor bridging the terminal; then Bob’s your Auntie and I’ve entered the rabbit hole…

In essence.
You will need to update the meters to change their default id of 1 to a unique ID.
(Not done that part yet myself though.)
You do this by sending a message to it over modbus, basically change your id from 1 to “n”.
You can cycle through the data on it meter - which includes its ID! Although labelling them would be a good idea :wink:

The Modbus RTU spec is 247 devices on one data link.

Getting that many devices to work without issue might be another story. :wink:

WRT bus termination… It may be needed, it may not.
Depends heavily on the length of the data link wires.

I’ve got two Elkor WattsOns on one bus - total wire run of about 10 meters.
I’ve no terminators on the bus. It has worked without a single issue for about 5 years.

Other forum users have said they’ve needed the terminators, and some have said their
equipment works OK without them. Some of that could be a result of using instruments that
have internal terminators.


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Wow 247 would be a LOT of meters!

Yes i can only assume that my usb rs485 adaptor doesn’t have any internal termination given that without the 150 ohm it just gives me garbage.
My run is only about 40cm so as always YMMV :smiley:

Another option to consider for your CU metering is using meters which emit pulses based on consumption. We got a bunch of them installed when we got the house rewired a couple of years back. There are 9 in total - lights upstairs, lights downstairs, lights kitchen, lights garage, sockets upstairs, sockets downstairs, sockets kitchen, sockets garage & cooking.

I don’t have the exact model of meter to hand, but they were something like this.

In terms of reading reading them, I use one of the USB devices mentioned here. I’m not sure if the guy behind the site is still operating, but there are presumably other versions available. The code for this device allows me to sum up readings before sending them to emonCMS for logging, so as well as the feeds above I also have things like all sockets (the sum of the 4 socket circuits), all upstairs (the sum of the 2 upstairs circuits), etc . . .

I only found out about modbus after the meters went in, but would likely have used it if I had known about it at the time.