Problems monitoring multiple, independent single phase mains supplies


I’m a newbie but have my emonpi setup and running, but Im finding that one of the mains supplies is ready way off - I’ve used a clamp meter to check the values being reported, and power1 is right but power2 is about half what it should be.

The figures are for power1 the clamp meter says 3.6A at 240V, and the emon reads 860W; power2 the clamp meter reads 5.8A at 240V and the emon reads 750W.

I suspect that the reason for this is that the two power supplies that I am trying to read are from two completely independent electrical supplies (ie each has their own meter and own bill etc). I do not know if they are different phases.

The emonpi and the 5V supply are both plugged into the mains supply on power1 - as I understand it, the 5V supply is used to calibrate the readings in some way, so as the 5V supply is coming from a different supply (possibly a different phase) that’s why the numbers are off.

Is this correct? And what can I do to solve the problem?


No, I’m afraid that’s completely wrong. The 5 V d.c. power supply is just that, it supplies the power to operate the emonPi. It plays no part in the measurement process itself. It’s the 9 V a.c. that is used (solely) to measure the mains voltage that’s used in the power calculations.

From what you say, they are different phases. If you use the wrong (voltage) phase to measure the current against, the power will be exactly -½ what it should be. This isn’t the normal set-up, I’m guessing there’s an historical reason behind it?

If you want to check - and beware this could be dangerous if you don’t know what you’re doing - measure the voltage between the Line conductors on the two systems. If they’re on the same phase, you might measure a few volts. If they are different phases, you will measure about 415 V. Hence the warning. If in doubt, don’t.

There are two things you can do. If measuring the real power accurately is important to you, you can either add an emonTx to measure the voltage and current of the second phase, or you can approach your electricity supplier or an electrician and request that both supplies are connected to the same phase.

If you’re not to bothered about real power and can tolerate apparent power, then you can modify the sketch in the “emon” part of the emonPi to report apparent power instead of real power for power2. The value is calculated, it’s simply not sent to the “Pi” part and therefore it’s not available in emonHub and emonCMS.

Hi Robert

Sorry, my mistake - I meant the auxiliary wall wart, so the 9V.

Thanks, but I’m not comfortable doing anything with multiple phases. But I think from what you’ve said that it’s on a separate phase, so I there’s nothing lost by not testing.

Yes - two houses put back together into one family house. Also, there used to be electric night storage heaters everywhere, around 90A in total so there had to be two separate supplies.

The storage heaters are gone now so we’ll be joining it back together, but thats not planned until we install solar sometime next year.

But in the mean time, I’ve unplugged the 9V and restarted the emon - the power readings are now much more what I would expect. Does this mean that I’m now measuring apparent power?

How accurate is this, without the 9V? I want to be able to chase the family to cut their usage, but most importantly this is for properly sizing a solar array we’re hoping to put in next year sometime.

(BTW sorry if this is going over old ground - I’ve tried reading pages like the AC Power Theory [1] in the learning centre but couldn’t see an answer)

[1] Learn | OpenEnergyMonitor

Yes, that’s correct - and with an assumed constant voltage as well - which (the last time I looked, it is 230 V) is the wrong value anyway for the UK.

How long is a piece of string? Without recording your voltage so that you know the variation, and without knowing the combined power factor of your loads, it’s impossible to say. If you can compare like for like - say weekly and assuming everything else remains the same, then it will give you some guidance. Otherwise, I’d suggest getting a cheap second-hand tablet or similar and put a dashboard on it in full view so that everybody can see the consumption as it happens.

Here’s my mains voltage over a period of 6 days. “0” is the nominal 240 V.

So can we make a stab at how (in)accurate it is? If your voltage is reasonably close to the UK centre voltage of 240, and your usage is spread evenly over periods when it’s either side of nominal, then you’ll be reading 4.16% low, provided everything is a purely resistive load. If your combined power factor is better than 0.96, then you’ll be reading up to 4% low. If it’s worse, you’ll be reading high. If you have something like a small induction motor, that on its own could read 25 - 50% high.

All I can suggest is over a period of time, compare the kWh totals against your supplier’s meters, and adjust the calibration (in emonhub.conf - via your web browser and “Setup”) to give reasonable agreement.

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Earlier today I went to add my email to to get the credits for my emonpi - it turns out that I bought an emontx back in 2014 that I vaguely remember! Time to see if I can find it…

Thanks for your help :+1:

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That could be the answer - first find it, then tell us which version it is. It could be a V3.2 (with the RFM12 piggy-backed on the RFµ 328 piggy-backed on the main board) or a V3.4.2. Either way, it’s going to be screen-printed on the bottom of the main board. Then it will be useful to know the type of radio.

(You do know that you’ve got emonCMS on your emonPi, and you can send the data to both that emonCMS and to if you wish?]

I’ll dig it out, the search might make me finish unpacking from our move last year…

Yes - I’m already doing it and have it showing up on the app :grin:

My plan is to record everything (electricity and heat) and add a dozen or so Pi’s with several DS18B20’s each to upload temperature from around the house, then later to replace the three timeclocks that control the boilers with nodered. It’s all based around emoncms and the MQTT server. I’m really pleased with how fast it’s coming together.

You can use a PiZero or a Wemos D1 and ESPHome (though there seems to be major issues with the ESP8266 WiFi library right now) as a cheaper solution.

There is also this… Alternative means of measuring room temperature Part 2 which is less energy intensive.

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Thanks for the suggestion, and I expect I’ll return to it in the future but I have a bunch of old Pi 2’s that I can get from work (we ship Pi based controllers, but don’t reuse older Pi’s that get returned). It’s not ideal that Pis are bulky and overkill for just sending temps and receiving relay on/off, but I’m a software developer with zero electronics experience to speak of, and so the Pi’s are a great (and convenient) way to get started quickly.

Much the same as myself. TBH, using the Wemos D1 and ESPHome is a doddle and more software than HW. Same with the Zigbee solution - I like that as it does not require lots of plugged in power supplies everywhere.

I was pointed to where you can get a Zigbee Stick ready to go - from there it is all software :grin:.

No worries. :smiley:

Robert is an EE with 50 years in the business, and I’m an ET with 45 years.
'tween the two of us, you should be able to “find answers to your questions.” :wink:

Found it - it is a v3.2.1 with RFM12b 433Mhz. I’ve got it running with batteries at the moment because there isn’t a socket anywhere nearby, so no 9V AC-AC calibration either - but at least the first supply has 9V calibration.