Where is the draw coming from

Hi, I have a emonTx4 connected to emonpi/SD monitoring our home 1ph supply. For years I’ve didn’t believe the usage, but couldn’t prove it. I’ve had check meter fitted, and had an electrician survey.

Now i have some decent data capture via emon , I am seeing some results I’m trying to understand, so am looking for advice

I have added a graph showing readings for the early hours, when hopefully demand is quiet.

There is a correlation ‘shed supply’ (some thermostatic heaters), and the calculated ‘sum’ channel as expected. This is also shown in the Dist Board Input(feed to Dist Board) BUT the reading is magnified as load increases. I am trying to work out how thsi can occur, so sagely advice welcome!

Welcome, Philip, to the OEM forum.

With the information you’ve given, it’s really hard to guess what’s going on.

Picking a time to read the numbers, say 3:40, I see:

Shed: ~120 W
DB Input: ~750 W
Sum: ~550 W

Are you expecting Shed Supply ≈ Dist Board Input, (or more exactly: Shed Supply + something in the house = Dist Board Input) and if so, what’s the significance of “Sum Usage”?

Which inputs are you using on your emonTx, which c.t’s (currrent rating) are you using on each, and what is the calibration setting for each input you’re using?
Where is the Shed Supply fed from, in relation to the Dist Board and your Grid incomer?

And how are you getting the “Sum Usage”?

Hi Robert,

Thanks for your response, so to answer your questions

I have the following config of CT’s on Dist board
P1 100A - Dist Board input from Supply Meter
P2 50A - Plugs downstairs
P3 50A - Kitchen wired sockets
P4 50A - Shed (Large garden shed with Freezer, and frost stat heater
P5 20A - Plugs upstairs
P6 20A - Lights upstairs

The Dist Board Input (P1) 'should be all incoming to the board, everything should be feed of this BUT I am trying to confirm as the meter readings to supplier are always high, and a number of trades people have commented its very high as we have no electric heating, little usage of electric shower(1 a week), Cooker is electric oven / gas hobs, and all lights are LED. My home office does have a server and other it ‘bits running’ but nothing which would be in the kW range. Water/heating is from wood fuelled stove with back boiler.

the ‘Sum Usage’ is a virtual channel adding channels p2-p6. I still have to get the pulse counter installed, as meter box is outside, Dist. board inside.

I haven’t got enough channels on the TX4 to monitor all circuits.

I am suspicious of the house wiring, as it was health board, then separated into private housing, and 2 units knocked through. I have found questionable wiring over the years, and have had an electrician look over it, and attempted to take usage reading, but very inconclusive as results were more ‘instantaneous’ using current clamp meter

My 1st level of sense check is to confirm input to Dist. board = Output to circuits, and also compare input to dist. board to meter reading via Pulse counter(I know it’s not the same units ) in case there is a hidden/phantom supply to somewhere else in the block. However, when I started looking at the graphs I spotted the ‘pulse’ from the shed circuit’ being magnified, rather some other ‘delta’ from unknown demands.
I know this has been longwinded, but the need is to explain the situation, and why I have setup the emon units, and hopefully get some answers, and look at reducing my huge electricity bill

If you are able to isolate down to a circuit level it may be worth getting some energy monitoring smart plugs and going per device over a 48 hour period (as fridges and freezers will have a transient demand and not be always running at full power).

If you have an electrician available you could always look to get modbus meters installed (sdm120m/sdm230m) as you can daisy chain these into a usb rs485 adapter and it can be read by the emonhub (to give you better visibility).

As these would need to be installed in line with the circuit this is something you may want to leave to an electrician.

I immediately spot a 50 A c.t. (No.4 - Shed) calibrated as a 20 A - a factor of 2.5 meaning the shed load is recorded at 40% of what it is. You need to change this in the emonTx. Everything in emonCMS (e.g. this c.t. plus the sum) will be using the wrong value for CT4.

But the emonTx4 does make energy units available (‘E’ values - in Wh) and barring static offset due to the different start times, the ‘E’ value should have the same slope as the pulse count from your supplier’s meter, once you calibrate that for the number of pulses per Wh.

That’s not unknown. Many years ago we had a member with exactly this problem who bought OEM monitoring and proved this was the case. He received a substantial repayment for electricity he had been wrongly charged for.

You can add the expander board to take it up to 12 power/energy inputs.
Or, of the slack in the wiring allows it and they’re all the same phase (or you have the 3-phase emonVs), you can group circuits on one phase through one c.t. and “physically” sum the currents in the c.t.

Hi Robert,

Thanks for the ‘catch’ of the CT 4 calibration, usual case of looking too hard.

After my previous email I went back to inputs and read up on the ‘e’ channels.

I will look at adding an extender, as I want to monitor on a ‘per circuit’ basis

Thanks again, I have reset the calibration for CT 4, and will now wait to see what results I get from here on

Hi Zak,

Thanks for your response, I have Ct’s per breaker at the moment, and am going to act on Roberts advice to look at an extender board and more CT’s. If I need better resolution, then I will look at your suggestions


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Just to confirm, you did upload that config to the TX4?

Is the TX4 connected by USB?

Hi Brian,
The config of CT has been uploaded, on Richard’s great catch, correct the calibration on CT4.
Yes this was uploaded by USB.

Note, correcting the calibration, it is now showing more acceptable data. I can continue to dig into the consumption details