CT accuracy

I currently have an OWL PV system it is currently reporting we are generating 27w and consuming 222w from the grid.

In comparison our luxpower charger/inverter is reporting 2w from the PV and 0w to/from the grid.

In reality we are using 48w (from smart meter)

Are the CT clamps which come with the the EmonPi more accurate than the Owl ones?


You can use whatever current transformer you like with the emonTx/emonPi, provided that the VA rating is enough to generate 1.1 V rms at the maximum current you want to measure, and you’re prepared to change the burden (a resistor on the p.c.b) if necessary. So you can use a c.t. to whatever accuracy you are prepared to afford.
However, it is inherent in the theory of current transformers that they must become less accurate at low currents. Therefore, you should always use the c.t. with a rating that is as close as possible to but greater than the maximum expected current.

But also bear in mind that (to the best of my knowledge), the Owl only guesses power based on a constant voltage (which you should set) and a presumed power factor. The emonPi calculates real power based on voltage and current samples.

For what it’s worth, I’m presently testing emonLibCM. After carefully calibrating, it’s agreeing with the pulse count from my non-smart meter to within 0.04% overall, but there is a detectable difference depending on the current itself - due to the phase errors of the YHDC c.t. being current-dependent. It’s those errors which become smaller, the more money you spend on the c.t. And don’t forget the v.t. either - that too has voltage-dependent phase errors that contribute to the overall error.

So out the box as it uses real power based on voltage and current samples it will be more accurate than the Owl?

I wouldn’t be so categorical as to write “will”, but it certainly should, especially if you’re using a range of appliances with different power factors, over a range of supply voltages. (For instance, I recorded 244.9 V at 6:15 and 232.8 at 20:48 yesterday.)

WHAT so the mains in the UK is not always 240v :wink:

And yes working in IT I understand the use of should :wink:

Believe it or not, there are a good few people who believe it is. And those who believe it magically changed to 230 V - and I suspect they’d believe it if you said it would go back to 240 V at the end of this month. If you search the forum, I remember one EV user who had to put the kettle on in order to start charging his car - if the voltage was too high, it refused to start charging!

Any data on just how far out it can get, while still maintaining 0.04% overall?

Any plans to experiment with dynamic phase correction?

Not immediately - I’ll try to dig some out.

Yes, emonLibCM is well suited to that, because the phase compensation is added at the end of the measurement period, when the average current is known. It’s not trivial, but nothing much will need changing and I don’t see any major snags. So there are plans. Just don’t ask when they might see the light of day. I don’t have firm ideas about exactly how I’d implement it, but it will probably be 5 segments: constant below point 1, interpolated between points 1 & 2 (the side of the ‘bathtub’), between points 2 & 3 (the normal working range), between points 3 & 4 (heading into overload) and constant above the highest point (4). Those would be a 2 × 4 array. That won’t be totally accurate, but it should be a big improvement.

A rough value is ±2%.
My overnight consumption at an average of 70 W (that’s about 40 W of router, cordless phone, PVR on standby and intermittently about 100 W of fridge/freezer) shows the measured/calculated real power 1.9% ahead of the meter’s pulse count. On a 3 kW heating load, it lags 1.75%.

Or possibly more realistically, you could say that the c.t. is reading high by 3¾% on very light (and probably lowish power factor) load.

[I’d say that’s not bad for a c.t. whose accuracy is only specified down to 10 A (~ 2.4 kW).]