How many circuits do people monitor typically

Hi All.

I really like what this system does and so before dipping my toes in I would lie to understand how far people go with monitoring circuits. A typical consumer unit will have way more than the 6 available CT sensors can measure. Do people people just measure the main plus a few of the more important circuits or do people go all out and measure all circuits by using the add-on module.


Welcome, Aaron, to the OEM forum.

The emonTx V4 - which is what I assume you are referring to, other emon** and even emon**** devices are or have been available - has only been on the market for a very short time, and the expansion module isn’t yet on sale. So I think all you’ll get is hopes, not experience, in terms of the V4.

If you search the forum, you’ll find that there’s a big spread of what people want from their system, some are happy with just monitoring the ‘whole house’, some want the split between grid and PV, some want to monitor each circuit and some want to monitor each appliance.


I think the nirvana is to monitor them all, with the 12 CT option, that is probably possible for the majority of households. The problem then is the CU has not been wired in a CT friendly way.

It also depends on why you are monitoring. After a while, the use settles down and you take less notice of it on a circuit by circuit basis.

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I was also thinking that after a while then certain things become less important to monitor and others more so. Thanks for confirming. I think that for me the important measurements will be around heating and Hot water. Things like lights and ovens etc will just be normal use. Outside and indoor temperatures as well so can see loads vs temps.

I have two emontx4 devices - one in the airing cupboard monitoring power usage by my heat pump and immersion heater (as well as flow/return temperatures), and another one by the consumer unit which at the moment only measures input from the grid, but will monitor solar when they get installed and hopefully monitor the batteries too (not yet figured that out). I monitor room temperatures with Shelly Plus H&T devices (via Home Assistant), and use a smart plug from LocalBytes (again, via Home Assistant) to monitor usage of individual devices that I’m interested in (tumble dryer, washing machine, dishwasher, fridge, etc). I could probably start monitoring the oven circuit via the emontx4 for its usage.

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Hey @IanHarris that really helped me think about how multiple units can be deployed. I was always thinking that it had to monitor the consumer unit but it doesnt. It can monitor any power cable.

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Hey @IanHarris does that mean that you need two measure the voltage on only one unit or do both units need to measure the voltage. I assume that only one unit would need to do this and the base unit then uses this for any power calcs

I made this mistake myself - you need to measure the voltage on all emonTXs because the calculations are done on the emonTX before sending them off to their final destination.

you need to measure the voltage on all emonTXs because the calculations are done on the emonTX before sending them off to their final destination

I wonder if that could be improved upon by the team for v5. It would be nice if only one unit or even the RPi base unit measures the voltage for the calcs from all units

I believe it could be done with a cable between two emonTXs, but I suspect most users would have multiple emonTXs in locations where running such a cable between them would be impractical. And the voltage is needed instantaneously for the adjustment of the power to the point where over the air updates rather than the wire would be inaccurate. The difference in voltage from one instant to the next can be quite pronounced.

@IanHarris do you have the two voltage graphs overlaid to show if the voltage is measured the same or differently by the two units at the same point in time

When you consider that the INDIVIDUAL voltage and current samples are multiplied to give the instantaneous power, then averaged to give the real power - and there are in the order of 30 samples (or rather, sets of samples) per mains cycle, there’s a lot of data that needs to flow.

Take the emonTx V4 with 3 10-bit voltages, Trystan had hoped for around 2000 sample sets per second, that’s (sending 16-bit values, no checksums) 96k bps - to be handled at both ends and the delays due to processing accounted for. And don’t forget - a time error looks like a power factor error in the load you’re measuring, so at the receiving end the samples local and remote need to be synchronous to better than 1° electrical - at a p.f. of 0.2, that would turn into a p.f. of 0.18.

If you want to get it going, you’re welcome to try.

But if you only want apparent power - then it’s dead easy because you’re then talking in terms of tenths of seconds at the most - more likely an average over the 10 s reporting period that most people seem satisfied with.


thanks for the detailed explanation. It makes sense now.

Both my emonTX4 voltages log to the same feed rather than seperate feeds, so they are pretty much seen the same by each other.

If this is the case - you’re gaining nothing. The last one to have its data arrive in the time “slot” called the feed interval when you hover your mouse over the Feed, overwrites the earlier one. So as the two aren’t time locked, it’s anyone’s bet which one gets stored.

As others have said it depends what you want to achieve, in my case it’s behaviors in the house, a few examples of this showing where the data comes from.

My system is pretty hybrid based on an emonPI with 2CT’s (solar PV feed and DNO supply) and a range of other inputs that are not just core emon products.

  • I look at instantaneous whole house use from emonPI when in kitchen and ask “Why are we using xxx W” >>> family dispatched from kitchen to turn off things they have left on.

  • In the last year we have imported X kWh that could have been avoided by running the washing machine when the sun is out, solar from emonPI and the washing machine use from a Kassa smart plug. (there are some Pythonkassa API scripts can query it and write to the emonPI via the API)

  • How much energy has gone into the hot water Solar PV immersion divert, I have a home made current sensor on the feed to the diverter (Current clamp and Arduino that feeds a Rasperry PI that then feeds the emonPI) it only measures current but as there is only the diverter on the circuit I simply graphed the current-power relationship and make a look up table on the PI. The same Raspberry PI records a few temperatures on the hot water system. I have to admit I only made this myself as there were no emon products available in the shop last year - although I learned loads doing it!

  • What’s the outside temperature? I have an existing weather station that uploads data to weather underground, they have an API, again I have a script that calls the API and then writes the weather data to the emonPI via it’s API. The one downside of using cloud providers is that if there are internet problems you have lost your data.

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Do you have any issues transmitting through walls etc. My new house is planned to be a passive build so airtight but I would like outdoor sensors. I have concrete and stone floors / walls so am concerned about how to get a signal to the base unit. It is possible to put a cable through the envelope at build time and seal it with tape.

There are all sorts of things that need to go through the fabric, outside light cables, internet/fibre etc.

I put some round plastic conduit through the fabric, sealed that then pushed cables etc through then sealed the pipe with silicone.

Try and put your gas and electric meters as close as possible together! If not the ‘smart’ meters will never communicate! If I built a house again, I’d put both outside and put a 13A socket in the outside box.

No gas, just electricity. I believe in NZ the consumer box needs to be inside the house. Its on a hillside so HW cylinder will be under the house and has 3 heating elements to heat 500 litres during free hours of power. Was thinking of having one unit outside to measure the 3 independently controlled supplies to the HW plus an outside air temp thermostat, plus cylinder temperature. Then another unit to measure all other circuits inside. The floor is two layers of concrete and lots of insulation between. So if I cable it, can I run a long USB cable.

I second that. Put pipes everywhere you can think of, then some more. The idea is an empty pipe is future-proof - if not totally, then at least more so than cables are likely to be. Cap them if insects or rodents are likely to be a problem. Seal with a weak mortar mix if you’re worried about the risk of fire or smoke spreading.

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