Jump to content

Energy metering - reliability of Goodwe Smartmeter


markus_m2
 Share

Recommended Posts

Hey guys,

So this lockdown has given me time to closely monitor the energy of the newly installed PV system.

Straight to the point, I've got three energy meters measuring the municipal power usage:

1. Goodwe smartmeter as part of the Goodwe install.

2. Efergy Engage (basic ct clip)

3. Municipal meter (old rotary type).

 

Comparing 1 to 2:

The near instantaneous readings are out by a large margin, and more confusing is that the difference isn't constant - Efergy reads anything between 0% and 80% higher than the Goodwe.

Typical reading when house is sleeping:

Goodwe: 170W  /  Efergy: 245W

 

Comparing 1 to 3:

Municipal meter reading over time around 10% higher than Goodwe (comparing exact same time frame 20 days).

 

So my question is, which of these meters is the most accurate?

Goodwe smartmeter measures usage based on accurate voltage reading and ct clip amp reading. How accurate are such ct clip measurements? 

Municipal meter - would have expected this one to be accurate, but who knows? Old rotary type, could this be "off" by 10%?

I don't expect the efergy meter to make top accuracy list 😉 For one it only measures amps (assumes 240v)...but still, one wouldn't expect it to be that far off (up to around 80% higher than goodwe). 

Maybe ct clips don't measure power factor "lost power" compared to the municipal meter?

Goodwe ct clip correctly installed (correct orientation and also directly onto mains feeding into house) - is there any further "best practice" when installing the ct clip? Example: keep as far as possible from other cables, if possible let cable run through the middle (rather than just hanging the clip on the cable), etc? Or does it suffice to simply just clip around the cable, even if the negative or other cables run right next to it?

Thanks guys, appreciate any input from those in the know...

 

Link to comment
Share on other sites

9 hours ago, markus_m2 said:

So my question is, which of these meters is the most accurate?

 

9 hours ago, markus_m2 said:

I don't expect the efergy meter to make top accuracy list 😉 For one it only measures amps (assumes 240v)...but still, one wouldn't expect it to be that far off (up to around 80% higher than goodwe). 

Exactly for this reason, you should not look at Efergy at all. 

9 hours ago, markus_m2 said:

Municipal meter reading over time around 10% higher than Goodwe (comparing exact same time frame 20 days).

In general throughout the electrical industry,  if you use any type of meter for billing purposes it should be within a certain accuracy class (0.5% if I remember correctly). The spinning disc meters use to be very accurate although i did see some faulty ones in my life that needed to be replaced. 

On the other hand, most electronic type meters with clamp on CT's use the 100 amp CT, from playing around with this CT in some electronic projects, it became clear that it is not that accurate way down on the low side of things, and some of them only start giving readings at 100-150 watt loads. The Spinning disc will read the low usage better in my opinion, and this might contribute to the difference that you see. 

Its difficult to say which one is more accurate without the use of a calibrated tester to compare the true readings. 

Link to comment
Share on other sites

2 hours ago, Jaco de Jongh said:

On the other hand, most electronic type meters with clamp on CT's use the 100 amp CT, from playing around with this CT in some electronic projects, it became clear that it is not that accurate way down on the low side of things, and some of them only start giving readings at 100-150 watt loads. The Spinning disc will read the low usage better in my opinion, and this might contribute to the difference that you see. 

It comes down to the range of the ADC used to read it. For example, if you use an ATMEGA328 chip (aka arduino), you have a 1024 bit ADC, which means you can measure about  thousand distinct levels of current. If you use a 100A CT and you want the full range, that means you can measure in steps of 100/1000 = 0.1A, so your smallest increment is about 250W, and the slightest bit of disturbance can cause small differences in measurement which means you can be 250W off easily (which is why you are advised to read the ADC more than once and take an average when you write code for this sort of thing).

What plug-in meters (like the Kill-a-watt and some efergy-branded units) do, is they have two ranges and they switch between them. At low power levels it uses one circuit to do the work (this one is more sensitive), and as the power levels go up they switch to a higher range that is less accurate. This is plug-in meters, not to be confused with the ones that use only a clamp-on CT. The clamp-on types are the least accurate because they don't measure the voltage, and they don't consider power factor.

Municipal meters can (with age) become inaccurate. You can then request that the municipality test it. In Cape Town at least, if you are right (and it is faulty), the test and replacement is free of charge, but if you are wrong, you get billed for the test. Of course, it might be better to just ask for a prepaid meter. Often municipalities will do that for free.

I don't know the Goodwe meters that well, but my experience with the Carlo Gavazzi meters is that it does read different to the municipal meter. In the old house I had a prepaid meter with a red LED that would blink every 1 Wh (1000 flashes = 1kWh). The Carlo Gavazzi's LED also blinks at the same rate. So you would expect them to blink sort-of in tandem.... which it does.... but over the space of some minutes you will see them drift into- and out-of phase, clearly indicating they don't quite measure the same value.

I unfortunately never did a proper comparison.

Link to comment
Share on other sites

  • 1 month later...

Hey guys,

Just a quick update on the above:

I had an old pre-paid meter lying around which I re-commissioned and wired into my panel for 2 weeks for testing.

End result: Goodwe Smartmeter with CT-clamp is about as accurate as can be!

Efergy Engage CT-clamp meter is completely unreliable - I could live with 5%, maybe 10% out, but the Efergy over-reads by anything between 10% up to 100%!

Municipal Meter reading slightly high, may get them out to replace at some point...

 

 

Link to comment
Share on other sites

Something else I learned in the mean time, is that at really low power values your signal to noise ratio (SNR) gets really bad. To understand this practically, imagine that on that ADC scale of 0 to 1023, you should read a zero (but noise causes you to read a 1), and imagine now that the range was wide enough that 1 means 250W... well you end up measuring little 250W noise spikes when there is no load... 🙂

 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...