emonPi CM Firmware Update Oddities

I recently updated my old emonPi to the latest CM firmware (from the original Classic firmware), The process was a little fraught…but over the last few days it’s seems to be working again (although sadly I did manage to inadvertently delete my old histori feeds…but c’est la vie!)

What I have noticed is the emonPi now to be measuring much higher power readings, and also a higher Vrms reading. I guess I shouldn’t be surprised at some difference, given the continuous nature of the new monitoring…but the readings are definitely much higher than I expected.

During the last Octopus Energy Saving session (29th Nov, 5.00pm-6.30pm), Octopus seem to think I used 0.29kWh, 0.26kWh & 0.26kWh of energy in each of the three 30 minute blocks. This is quite close to what my emonPi was reading with the old firmware (not exacty but the emonPi monitors the house, and in the garage there is some unmonitored circuits with a small amount of energy consuming equipment (garage door opener, maglock power supply, external light sensors, for instance). I don’t remember ever calibrating the old firmware…and the monthly cumulative power readings were within a couple of kWh of the bills each month (which I attributed to garage power usage).

With the new CM firmware, this is the power usage measured in the Savings Session window yesterday

The new firmware seems to be measuring about double our meter measured usage, and about double what I would have expected using the old firmware (over a number of years of using the emonPi with old firmware, I have a pretty good idea what the house energy usage is like with most devices turned off)

On the Vrms side…over a year the old firmware measured our mains voltage mostly around 232V, and nothing higher than 238V. Here’s a histogram of a year worth of readings.

With the new firmware I seem to be seeing a higher average voltage (although with just a couple of days readings that’s probably not a great sample population)

Is this behaviour expected in a move to the new CM firmware, or maybe there a default calibration or factor applied somewhere which I may have wiped out when re-configuring the new firmware in the emoncms.cfg file?

Any thoughts around what might be occurring?


Hello @rdavies6

I haven’t noticed this, but will do a quick sanity check test on an emonPi board here to see what it gives.
I wonder if it’s loading an incorrect calibration from the EEPROM…

It might be worth opening the Admin > Serial Config utility, stopping EmonHub, connecting to ttyAMA0 at 38400 baud and then first list the config with l and then resetting the config with r.

Is this what you expect to see (i needed to connect at the higher baud rate too)

Is there a chance that the type of CT clamp that’s being used is not correct in the new config? (That means I also have to figure out what the type of CT clamp I got with the emonPi was… :smiley:)

A quick test here and Im reading 247V on the emonPi vs 242V on an SDM120, and 728W vs 715W.

It might be worth adjusting the voltage calibration a little, e.g send via serial:

k0 263.52 

Save with


That brings the voltage and power in line with the SDM120 reference here, but doesn’t explain your 2x error?

I’m testing with an Ideal Power DB-06-09 power adapter here, what do you have there? Is your CT sensor SCT013 100A:50mA?

Why do you say that? Is it me going barmy, or looking at the last hour is 0.26 kWh over ½ hour not 520 Wh per hour, or 520 W? I think that makes your emonPi reading at roughly 500 W about 4% low, not 2× (high).


Argh…it’s very possible it’s me going barmy then! Such simple maths… :pleading_face: Sory