OpenEnergyMonitor Community

Power reading about half of true value


I have an older Emonpi that I have recently got going again after years of in being pack away.

I have 2 ct clips connected at the fuse board.
No. 1 on the neutral coming from the main RCD (easiest to get a good connection on)
No. 2 on the live for the main socket ring main (both ends)

Both CTs are reading approximately half of the value I would except.

The sd card has emonSD-30Oct18 on it and hasn’t been messed with yet by me. so I assume all the constants are correct. (can’t seem to log in via SSH on my Mac to check. bit of paint to log in locally)

Any ideas?


Starting with your version, SSH is off by default. See here to enable it: emonPi SSH disabled by default

Now the power readings.

Do you have the standard UK single phase supply?

Have you got an a.c. adapter? If so, is that showing a roughly correct value for the voltage in emonCMS. If you haven’t, does emonCMS show a constant 230 V?

Are the c.t’s properly closed around the cables - there’s no dust or dirt on the mating faces of the cores? Are the plugs on the c.t. cables fully home?

Those are the obvious things to check. Beyond that, I can’t say that I can recollect seeing this particular problem before, and I can’t immediately think of anything - assuming you have a “factory” version of emonCMS - that could cause the result you are seeing.

Hi Robert,

thanks for your reply.

The first problem of ssh was easily fixed. Thanks.

Uk with voltage from ac plug readying about 229 to 230v today
all readying from emonpi.local/emoncms/
I have check both ct’s. looks good as far as I can tell. I suspect they are fine as there don’t appear to be any randomness in the readings. if I use the kettle both readings go up by the same amount.

I have a emonTH connected which is working fine.

emonhub.conf looks good. is there anywhere else there could be a wrong constant? the error looks to be exactly half the true power value.

Sorry, I forget to say “Welcome” to you.

I should have asked another question last time: Which make & model of CT do you have?
If you have anything other than the YHDC SCT-013-000 from the OEM Shop (or elsewhere), then the calibration is going to be wrong. (Note: the SCT-013-000V would give you exactly half-power, because that one has an internal burden resistor, so you’d have two burdens hence half the current in each.)

There are two places where there are “calibration” constants. The real calibration is done in the front end of the emonPi, in the “emon” part. This is actually a stripped-down, 2 channel emonTx in most respects, so everything about how the emonTx is calibrated applies to that.
There’s a second place where you can do a partial calibration, that’s in emonhub.conf (accessible via the web browser and the emonHub page). It wasn’t intended for calibration, so the full calibration isn’t available there; but emonhub.conf is easily accessible.

My first thought was you’d got “USA” mode set. This changes the front-end voltage calibration to read 120 V instead of 240, and uses that in the power calculation. But as you’re seeing the correct voltage, that’s not it.
The second thought was you had a 3-phase supply, and if the a.c. adapter was on a different phase to the c.t., you’d also get exactly half-power with a resistive load, and probably nearly so for most other things. But few UK dwellings have a 3-phase supply (though that might change as everyone gets EVs).

And if you’ve not changed anything in emonCMS (including emonhub.conf), I can’t see the calibration in the ‘emon’ part being wrong. And besides, it’s quite hard to get at - except for the ‘USA’ switch.
You can easily check the scale constants in emonhub.conf, the set of values for the emonPi itself looks like this:

    nodename = emonpi
        names = power1,power2,power1pluspower2,vrms,t1,t2,t3,t4,t5,t6,pulsecount
        datacodes = h, h, h, h, h, h, h, h, h, h, L
        scales = 1,1,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
        units = W,W,W,V,C,C,C,C,C,C,p

Yours might not be exactly the same (that’s the 2019 version) but all the “scales = …” values are either 1, 0.1 or 0.01, and you’d need a “0.5” in the first two or three to get half-power.

One thing I would strongly advise: update your emonCMS to the present version, emonSD-08May21. We can’t maintain “old” versions indefinitely, and as you’re just starting, you won’t have the problem of migrating your data when you’re forced to update later on. I have the latest running on a RPi 2B (it’s just a bit slow showing a big graph), so the age of the emonPi isn’t a problem.