How accurate are cheap mains power meters?

[TheUnnamedNewbie]

So I am looking at upgrading a bunch of networking stuff in the house. For some things, I'm not yet sure that I will upgrade them, and it depends on how much energy it costs me to run them.

In order to figure that out I wanted to get myself a power meter, one of those plug-in ones. However, they seem to be too cheap to be reliable. What I was wondering was:

- How accurate are they?
- True RMS? (I'm gonna guess no?)
- Does the switching nature of IT equipment give nonsense results on these meters?

Any tips? Worst case, I would just build up a fixture for my K2000 so I can measure it for a while and log it.

[capt bullshot]

Some of them are pretty good in terms of accuracy, others are real crap.
One cannot tell the difference from price or look of these things. One of the best ones I have did cost least and looks very cheap.

Speaking of their guts: The good ones use a power metering chip, the bad ones try to measure something using other ways. Often the good ones have less display modes, this is due to some of these metering chips output only one information: Energy consumption encoded in a pulse train. So the only thing that can be shown is Watt and kWh (and derived energy cost).
If you find a more sophisticated one that displays Volt, Ampere, Watt and whatever, it might be a shitty one or a good one (there are energy metering chips that output all these informations).
The shitty ones usually try to measure Volt and Ampere and calculate VA from the product of both, and have some more or less reliable mechanism to measure phase shift, then calculate Watt. These are easily fooled by SMPS like loads, but may show plausible values for incandescent lamps.

Edit: one cannot build a useful power meter from multimeters ... except you add some analog multiplier to your fixture (that multiplies line voltage and load current in the time domain, smooth the output, result is real power).

[MrW0lf]

reliable mechanism to measure phase shift, then calculate Watt.

Aha, now you opened can of worms. You cannot calculate power stuff from phase shift for non-sinusoidal crap! You can calculate power factor* though:
integral(A*B)/(sqrt(integral(A*A))*sqrt(integral(B*B)))
which is often wrapped as such
acos(integral(A*B)/(sqrt(integral(A*A))*sqrt(integral(B*B))))/Pi*180)
to give phase shift**, which will be incorrect for non-sinusoidal.

*

power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number in the closed interval of −1 to 1.

**

Phase difference is the difference, expressed in degrees or radians, between two waves having the same frequency and referenced to the same point in time.

Does someone have in depth info on this? Brief Google lookup did show some controversy around power factor on complex wfms.

[HKJ]

Some time ago I did test a couple of power meters, they where fairly precise, the main problem is very low power.

https://lygte-info.dk/review/Power%20Power%20meter%20TS-836%20UK.html
https://lygte-info.dk/review/Power%20Electronic%20Energy%20Meter%20TS-838%20UK.html
https://lygte-info.dk/review/Power%20Zhurui%20Power%20Recorder%20PR10%20UK.html

[joeqsmith]

I bought a Kill A Watt meter a few years ago and compared it with my DSO .

Video showing me using purely resistive and capacitive loads as well as a few different switching supplies.   There are also a few different videos where I show how I set the DSO up to make these measurements. 

https://youtu.be/ggnG8O1EL54?t=156

Spelling is rough...

[capt bullshot]

Does someone have in depth info on this? Brief Google lookup did show some controversy around power factor on complex wfms.

Disclaimer: maybe ten years ago, I developed "Power Quality Measurement Instruments", aside from other things like Flicker, Harmonics, Transients, these things incorporate a power meter.
So usually you measure:
- Urms and Irms over an interval of 200ms, and display as Urms and Irms.
- "Real Power" (Wirkleistung)  (Watt), as U(t)*I(t) integrated over an interval of 200ms.
- "Apparent Power" (Scheinleistung)  (VA) is just Urms * Irms.
The above are independent of frequency and waveform, for a line frequency power meter, the integration (measurement) interval is supposed to be a multiple of the line period (10 * 20ms or 12 * 16.7ms).

Now it comes to call kinds of power factors:
Calculate (from a FFT or DFT) the displacement power factor: that's what we call cos(phi). Applicable for sinusoidal waveforms.
From the same FFT / DFT one calculates the fundamental real and reactive power. Also applicable for sinosoidal waveforms.
To add more complexity, one can calculate all these power values for each harmonic, but this is usually not so interesting.
Fundamental means the nominal line frequency here (50Hz or 60Hz). Use a reactangular window for your FFT or DFT and synchronize your sampling frequency to the line freqency, so there's a integer number of samples within a 10 / 12 line periods window. Sample U and I simultaneously. From the FFT / DFT you get complex numbers as result, representing the amplitudes and phase relationship, from this one can calculate the mentioned values.

From the above (waveform independent) apparent power and real power one calculates the "Power Factor" - this is not the same as the mentioned "displacement power factor".

So short form:
If one says "Power factor" this can mean both "Real Power / Apparent Power" (independent of the waveform) or displacement power factor / "cos(phi)"  - (power factor caused by phase angle)
If one says cos(phi) or displacement power factor (DPF), this clearly means the power factor caused by phase angle. The "power factor" (PF) can be worse than the DPF in case of distorted waveforms.
Especially SMPS can have a near 1.0 DPF but a bad (<0.5) PF.

Don't expect this from a plugin-type power meter - the good ones display real power and maybe the PF (not the DPF), the bad ones might use the DPF to calculate the real power from Urms*Irms*DPF - but often quite unreliable and not suitable for distorted waveforms. And their Urms / Irms isn't a real rms but rather a simple rectifier and the form factor applied, so more crap for distorted waveforms.

[R_G_B_]

I have some single phase analysers. The problem I find is DC power analyser seem to be  rare.

The gossen metrahit energy can do D.C but only up to 10 amps using the meter its self.... for firmware versions less than 2.00.

with later version of firmware greater than 2.00 above  it can uses a hall effect current clamp with a shunt resistor to measure higher D.C power  greater than 10 amps.

There are many A.C power energy analysers of all types and they can be very expensive.

R_G_B

[R_G_B_]

not cheap, looks interesting though

http://wattsview.com/

[Gregg]

Big Clive likes his Hopi https://www.aliexpress.com/item/HOPI-HP-9800-Handheld-Power-Meter-Power-Analyzer-LED-Metering-Socket-Measurable-Current-voltage-Power-Factor/32375959317.html
I bought one, put a proper fuse inside, changed to a US power cord and changed the neutral at the installed outlet to conform to US arrangement and made some P-touch labels.  I find it more versatile than a Kill-a-watt. 

[coppice]

Some of them are pretty good in terms of accuracy, others are real crap.
One cannot tell the difference from price or look of these things. One of the best ones I have did cost least and looks very cheap.

Edit: one cannot build a useful power meter from multimeters ... except you add some analog multiplier to your fixture (that multiplies line voltage and load current in the time domain, smooth the output, result is real power).

Spot on. I've seen meters from some well known European brands merely measure Vrms and Irms and multiply the two. The Kill-A-Watt range all seem to measure properly and to pretty good accuracy, but they only seem to make products for the US market. There must be quite a few other decent products, but I don't know of any place you can look for trustworthy recommendations.

[bitwelder]

Big Clive likes his Hopi https://www.aliexpress.com/item/HOPI-HP-9800-Handheld-Power-Meter-Power-Analyzer-LED-Metering-Socket-Measurable-Current-voltage-Power-Factor/32375959317.html

Cleverly sorted by Aliexpress under 'Pressure Monitors' category 

[Gregg]

Big Clive likes his Hopi https://www.aliexpress.com/item/HOPI-HP-9800-Handheld-Power-Meter-Power-Analyzer-LED-Metering-Socket-Measurable-Current-voltage-Power-Factor/32375959317.html

Cleverly sorted by Aliexpress under 'Pressure Monitors' category 

I guess I took the easy route bypassing the Aliexpress hurdles and just googled "Hopi meter" and Ali was about the third result.  Now the big question remains, is it genuine?  The price seems quite good and there is probably not much market for fake Hopi meters; but it does come from China where anything goes.
If someone buys one and wants to compare photos, I'm game. 
Maybe we can convince Joe to properly abuse one for entertainment purposes. 
Edit: Mine hasn't shown any signs of over pressure

[_Wim_]

So I am looking at upgrading a bunch of networking stuff in the house. For some things, I'm not yet sure that I will upgrade them, and it depends on how much energy it costs me to run them.

In order to figure that out I wanted to get myself a power meter, one of those plug-in ones. However, they seem to be too cheap to be reliable. What I was wondering was:

- How accurate are they?
- True RMS? (I'm gonna guess no?)
- Does the switching nature of IT equipment give nonsense results on these meters?

Any tips? Worst case, I would just build up a fixture for my K2000 so I can measure it for a while and log it.

If you want to DIY, I have a MCP3905A Energy Meter Evaluation Board lying around(https://www.microchip.com/DevelopmentTools/ProductDetails/mcp3905ev#additional-summary) You can have it for free if interested.

[R_G_B_]

Or this


https://youtu.be/X5VJ3Y1h304

[HKJ]

Or this

It is probably correct, but it has nothing to do with modern loads.

[rsjsouza]

I bought a Kill A Watt meter a few years ago and compared it with my DSO .

I haven't seen that video Joe. Thanks.

I have the same unit and did some measurements with it as well, which are quite consistent with yours. The unit is quite accurate for its low price.

[coppice]

I bought a Kill A Watt meter a few years ago and compared it with my DSO .

Video showing me using purely resistive and capacitive loads as well as a few different switching supplies.   There are also a few different videos where I show how I set the DSO up to make these measurements. 

https://youtu.be/ggnG8O1EL54?t=156

Spelling is rough...

Although Kill-A-Watt only specify 1% or 2% accuracy, I've found them much better than that in practice over quite a wide dynamic range. I've never opened one, but I expect they use a commercial energy metering chip. Their behaviour is consistent with the use of one, with a reasonable quality production time calibration.

Your scope showed about 5% THD with the incandescent light bulb. Was that THD in the voltage waveform or the current waveform? If you are looking at the current waveform, which will show the distortion of the load rather than the line, you typically get 15% to 20% distortion with an incandescent bulb, because of the huge resistance cycles they go through at 100Hz or 120Hz.

[joeqsmith]

I bought a Kill A Watt meter a few years ago and compared it with my DSO .

I haven't seen that video Joe. Thanks.

I have the same unit and did some measurements with it as well, which are quite consistent with yours. The unit is quite accurate for its low price.

I modeled the scope's non-linearity and over sample the signals to tighten things up but you can only do so much.   

https://youtu.be/04I7nHA_HxM?list=PLZSS2ajxhiQBcHhIaGpmm9GyZQfrCzqkv