Setting up an emonPi - some newbie questions

Very much a newbie, so please bear with me … :grinning:

I have plugged my new emonPi in, c/w 2 CT sensors and an AC voltage sensor; I set up wifi access, and can now “see” it on my laptop via emonpi.local (wirelessly). All good so far.

Within the Emoncms>Inputs>emonpi screen there are various entries, such as power1, power2, power1pluspower2 etc etc. All inputs have a green 5 sec label. But when I try to rename (eg) power1 by a) ticking the radio button and then clicking on the edit button and then typing (eg) “House(Main) / Old part of the house” and then SAVE, after a short period the “seconds” field goes orange, and then goes red - presumably indicating an error. (The watts value is correct in that it agrees with the power1 figure that reappears.)

I watched the video tutorial 3/6 by, but whilst the UI is slightly different I can’t see what I am doing wrong?

Any suggestions?


I had a similar problem you might need to go to Admin → search REDIS section → press Flush button

Reload inputs pages.
Is important i found that you make the naming changes as 1st thing in the setup or you might get issues on the feeds after.

Important EDIT. I am new to Emonpi so take what i say with a grain of salt! if i am wrong someone might correct me!

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The input names need to be fixed to work (e.g power1 is what the emonpi posts to emoncms), but you can change the input descriptions and you can set your own feed names.

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Many thanks - that’s now sorted :blush:

Another question: How do I delete a Feed? I created a couple of bad feeds, but can’t seem to be able to delete them within the Feeds tab. If I click on the radio button and then on the Trash icon, a Delete Feed dialog box comes up; I can then chose Dear Data or Trim Data, but whilst both work neither deletes the Feed. But I can’t work out why :grimacing:


On feeds Page - > make sure you Check mark the feed you want to delete

This will change the buttons at the top, in which you can see a little trash bin icon for delete

clicking the Trash bin will open a dialog box
(if running emoncms up to 10.1.2 You must scroll that dialog box) Then you see a red button saying "Delete feed permanently

Be sure you only check ONE feed and have no other selected. As it might delete more than one at once. You can imagine how i know that :smiley:

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Thanks Gil - I hadn’t seen the scroll bar - but now have and have deleted the feeds I wanted.

Happy :grinning:

Yes sorry, new 10.1.3 rearranged the prompt.
Before it needed to be scrolled down a bit to see the delete permanently, at least on chrome browser.


I have now been able to create a couple of Feeds and graph the output.

Stepping back a bit, when I compare the Input “power1pluspower2” from the emonPi to the equivalent figure produced by my 4-5 year old “Owl” system ( Solar PV Monitoring | Intuition PV | The OWL ), the values are quite different. Anything up to ~20% this afternoon.

The Owl system doesn’t have the benefit of an AC Vrms feed like my emonPi, and relies on (I think, as this is getting the limits of my electrical understanding) a Power Factor figure that is inputted once during set up.

Owl say “… our unit [works to an accuracy of +/-7% which is] to be expected with any energy monitor - our unit is not a calibrated electricity meter and by definition is meant to only give an indication , not a measurement, of electricity consumption/generation - to assist in decision-making regards your electricity use.”

I don’t want to get too hung up on detail or accuracy, but I’m always keen to get things as correct as they can be.

Both the Owl & the emonPi have two CTs on the very same cables. Any ideas where this variance might be coming from?


Hi Julian,

What you’re seeing from your Owl, is expected behavior.

The measurment it’s making is an approximation of apparent power.

Translated, that means its accuracy is just as the manufacturer explained. i.e. it can’t and won’t
ever be as accurate as your emonPi, because as you have observed, it lacks an AC voltage sample input to make real power measurements, which is what you get billed for.

(the only case where its accuracy would be close to that of your emonPi is with a resistive
load. e.g toaster or kettle. But, that’s not likely to happen as the various electrical loads typically found in one’s home are a mix of resistive and reactive devices. e.g. motors and computer power supplies)

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Thanks Bill - that is now understood. I’m still a bit surprised at the size of the variance; with the emonPi’s do I need to “do” anything with the Vrms info, other than to have it as an Input? Does the Emoncms system automatically adjust power1pluspower2 for example, or do I need to tell it to?

I’m beginning to feel that I’ve been living in a slightly cloudy data life these past 5 years! :crazy_face:

No, it’s only there for your information. Voltage and current are multiplied internally to give you the real power readings, power1 and power2. It also adds the two power samples taken at the same time to give you power1pluspower2. If you do that later in emonCMS, in certain circumstances it can use samples taken at different times - which can be confusing.

BUT - I find it interesting to watch how the voltage varies over a day or week. It often comes as a shock to see just how much it varies - which will go a long way to explaining what your Owl reported.

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You mean something like this?!

Shocked me the range of variation in voltage at my location.
212 → 250volts very dependent on load as well. When my EV Car is charging at 30amps oh boy!
What i find odd though is the variation as well without local load. Of course load can be up stream on the powerlines.
Voltage adjusted is simply VRM adjusted to what the multimetermeter would show when reading from the socket, in my case is VRms - 14.7v for some reason not sure why vrms is higher than multimeter reader. Is there a reasonable reason for that?


Indeed there is. All the analogue components in the emonTx and emonPi are subject to manufacturing tolerances. The amount of variation from the nominal value is different for each component. For example, the a.c. adapter (EU version) had a manufacturing tolerance of ±5%. So immediately there might be an error of 5%. You can read more in the ‘Learn’ section: Learn→Electricity Monitoring→Current & Voltage→2. Emontx error sources→Sources of Error in the emonTx Voltage and Current Inputs

The correct way to calibrate is not to subtract a fixed value, but to multiply by a factor. In your case that would be about 0.936 You can do this either in emonhub or as you have done, in emonCMS. There are very many posts here telling how to apply the calibration in emonHub. If the voltage is that far in error, you should probably try to calibrate the power values also (because adjusting the voltage does not adjust the power - that calculation has already been done in the Atmel 328P processor).

I understand thank you for the explanation

I am trying to get familiar with all the “cogs” of the system before starting to mess with calibration.
I noticed however already that Emonpi vs a digital rail meter show different Kwh reads when charging the EV car. Easy to see as the EV is on a single meter as is on a single CT clamp. (difference of about 5kwh on on a 30kwh charge which is significant)

I will investigate the calibration process might be a bit complex to aproach right away untill i get familiar with all other aspects of the system. Also unsure if this calibration needs to be done again after each update.

I will read on it! Thank you

I understand that. The calibration should not be affected by an update.

I have now set up the emonPi (c/w PV bundle) in the house - two CT sensors, an AC Vrms & an RJ45 thermometer - created inputs, logs and feeds, and can graph the output. I will leave the emonCMS bit until later.

I have just wired up an emonTx3 in the garage, c/w four CT sensors, an AC Vrms, an RJ45 thermometer and a wifi module.

A couple of questions …

  1. Unlike in the help page Using the emonTx v3 with the ESP8266 Huzzah WIFI module - Guide | OpenEnergyMonitor (which seems to be based on an earlier GUI?) at no point did it produce an SSID for me to wirelessly connect to it; but when I look in emonpi.local the emonTx3 shows up - but with all of the right hand columns in red/NULL rather than green & data. I presume therefore that I have not completely sorted the comms? Any suggestions?

  2. The garage is fed directly from the house, which is ~60m away; have I done wrong in having an AC Vrms power Brick / feed in both? Would I be better off removing the one in the garage?

  3. I have a 4kWh solar panel system, which comes in to the garage; I use what I can but export any excess production. I am not sure whether I have a Type 1 or Type 2 system, so am not sure which CT inputs to use or exactly which cables to attach the two CTs to either. Solar PV - Guide | OpenEnergyMonitor Any suggestions?

Thanks in advance, Julian

Do you actually have an ESP8266? If the emonTx is within radio range - and I’m talking about the 433 MHz ‘native’ capability of the emonTx and emonPi, using the in-built RFM69CW radio modules and NOT Wi-Fi, you don’t need one. And if you don’t have one, that is the wrong place to be looking. You should be following the main guide:

At 60 m distance, the RFM modules should be able to communicate if there’s a relatively clear path, but if there are too many solid walls in the way, it might be struggling. I presume you’ve attached the aerials to the SMA sockets?

If you remove it, you’ll need batteries or a 5 V USB power supply to feed it, and you won’t read real power, only an approximation: you’ll actually read the apparent power based on a nominal 230 V supply, and as you’re in the UK, that’s almost certainly hardly ever going to be the true mains voltage. So no, you will be worse off. What was your thinking there anyway?

You have neither a Type 1 nor a Type 2 - or that’s what I infer from you having 4 c.t’s into your emonTx in the garage. I need to know more about what you’re measuring where to say how best to treat your set-up. The essential difference between Types 1 & 2 is that Type 1 measures the consumption and PV input and infers the import/export, and Type 2 does the opposite: it measures the grid import/export and PV input and infers domestic consumption. Pending your answer and what comes of the final question, you’ve got closer to a Type 2 but you’re measuring some of the domestic consumption as well, and it’s split between the emonPi and the emonTx.

Can you identify the PV infeed in the garage (the output of the inverter) and the grid connection at your main meter? If you can’t, post some photos.
Can you do a single line diagram showing your meter, consumer unit(s), any Henley Blocks, the connection to the garage and how the PV is connected?

I have an ESP8266 attached to the emonPi and another the emonTx3; my wifi network extends in to the garage via Apple Airport Express etc, and I have given all access points the same SSID and p/w etc. Works well for www and email etc. I had sort of assumed / hoped that the emonPi and the emonTx3 would communicate via this WLAN. And to a certain extent they are (in that the emonpi.local pages show both the emonPi and the emonTx3), but I can’t get the emonTx3 Inputs to turn from red to green (see screenshot).

I understand that an AC Vrms input will give me actual rather than apparent power, so my question should have been “assuming all on one mains meter, do both an emonPi and an emonTx3 require their own AC Vrms plugs, or can both use the info derived from just one?”

My solar panels come in to the garage, when there is a CU and a generation meter; the garage is connected to the house’s mains circuitry, with the Utility Co’s meter is located. Any generated power gets used locally, with the rest being exported. At the moment I have x4 CTs on cables within the garage, but am aware that I need to re-jig them to fit in with emonPi’s PV monitoring systems. But before I can do that I need to understand what system I have (in emonPi’s terms) :grinning:

Hmmm, this might get too complicated, in that the SWA PV feed comes in to the garage, through a Generation Meter, then in to a CU which has various “contacts”, including another SWA that goes to the house and the main (utility co) meter. Does the fact that the input feed and the utility meter feed are in different buildings cause an issue? I had hoped that this is where the emonPi system would beat the OWL system, as that is more limited.

I attach my current diagram sketch - ignore the Office section as that is a) on a separate meter, and b) Phase 3 :smile:
2019-08-11 emonPi layout v3.1.pdf (738.3 KB)

That answers one question but poses another two.

What is your reason for the ESP8266 on the emonPi? The emonPi (if it’s a modern Pi) has internal WiFi and an Ethernet connection.

I.E, they are not updating. Do you also have a 5 V d.c USB supply to the emonTx? That is necessary if you have an ESP8266, because WiFi needs far more current than the a.c. adapter can supply without damaging the shape of the a.c. wave it’s measuring. Without it, the emonTx will fail to operate. That could explain the absence of a set-up page.

If you need to know real power, then yes they do - because the calculation is done some 50 times per mains cycle, and you can’t transfer the data between them that quickly. So they can’t share one. If you’re content with apparent power only and not knowing the direction of power flow, they can share the voltage, but you then need to correct the “power” readings by scaling from the assumed 230 V to the actual voltage.

By not measuring the garage power at the house end, you’re not accounting for cable losses, but my guess - and it’s only a guess - is that may easily be outweighed by the inaccuracy in having apparent power only. If you have the a.c. adapter in the garage, you can account for cable losses (but you might need to carefully calibrate the whole system to measure it accurately).

The way you’ve drawn it, you have a Type 1 system - but instead of 1 c.t. as shown in the guide, the “CT1” shown there needs to be the sum of your c.t’s 1, 2, A, B & D. “CT2” is your c.t. C.

Therefore, on the 'Inputs" page of emonCMS, you’ll need to add those 5 together into one feed, and c.t. C will be another. Once you get past the “quick” setup, the manual setup in the guide gives details of how to name those two resulting feeds and configure emonCMS - and that’s what you need to look at. What you do with all the other inputs is at your discretion, but I guess you’ll want to separate out the EVs, for example, and record their consumption separately.