Single Phase Power Meter - how many digits we need for Power Factor ?

[Kiriakos GR]

Single Phase Power Meter always was an instrument this having a price that home users and hobbyist refusing to pay.

Even today an benchtop single Phase Power Meter (branded) this is at above 1000 Euro mark.
Most people will end up getting the Chinese version of the small power meter that you do plug-in at your wall mains plug, and at the price of 12 ~ 15 Euro, you become capable to measure energy.

But there is a catch, especially for low watt and measurement of standby power.
The low cost power meter has the ability to measure Power Factor at only three digits resolution.

When the real full size power meter this has five digits for Power Factor.
Power Factor range this is from 1.0000   down to 0.0000

Cheap power meter, measurement example.

Volts= 235
Current = 120mA
Power Factor = 0.02
Power result in watts = 0.564 W (according web calculator)

Power meter displayed value 1.1W (1W)

In this example the measurement this includes a huge error.
Voltage and current values both are correct, I did confirm that by high quality multimeter.
This Cheap power meter has poor resolution at Power Factor measurement, and therefore for measurements of 5 Watt and bellow this cannot be trusted.

Why I did start exploring Power Factor and digits resolution?
I have one CORSAIR CX750 power supply for PC.
With the cheap power meter I did try to compare standby power, against a much older power supply this be also 780W.
The old PSU it was measured and it consumed 2.8W as standby power (technology of 2008).
The CX750 this is technology of 2013, and this was measured and it consumed 1.0W as standby power

CORSAIR datasheet  mention standby power of <0.45W

In conclusion, I do now feel lost and this is now justified.
a) The cheap power meter this having poor in resolution power factor measurement, and it is unable to measure low watts with out huge error.
b) The displayed measurement of 1 watt this include 65% of error.
c) Even the 3W measurement of the older PSU this is for me hard to believe.

Lesson learned, the cheap power meter it might be trusted for loads of 5W and above.
And also for PSU standby Watt measurements,  you need the real thing, an true Single Phase Power Meter with five digits for Power Factor

[Kiriakos GR]

By a quick look at the market of portable Power Quality and Energy Analyzer.

For example the FLUKE 430 Series II ( products release date 2012~2014), this offer Power Factor at 4 digits resolution.
4 digits resolution this translates to more accurate measurement of low watts, but not of standby power.
FLUKE 430 Series II this is designed to use external current clamp sensors, them adding also an error due their own circuits.

And therefore even these portable and quality made tools, they are not a substitute , of the class of power meters them designed specifically to measure low watts and or standby power. 

430 Series II  |  Power factor (Cos j/DPF) | 0.001 (resolution) | ± 0.1 % @ nominal load conditions

[overvolt]

CORSAIR datasheet  mention standby power of <0.45W

While the cheap power meter it might not be able to deliver qualifying measurements at measuring accurately of low Watts,  I think that CORSAIR datasheet  this offer specifications based at PSU this not be connected in to a motherboard.
And therefore the measurement of as Low as 1W, it might not be far away from the real standby value in watts. 

I do understand your point, that with out accurate delivery of Power Factor from the meter, the entire calculation at Low Watts, this is in significant error.
And as such, your entire testing and conclusions, they are 99% valid.     

[Andrew-88]

And as such, your entire testing and conclusions, they are 99% valid.     

Kiriakos as always, he does the best possible to prove a point. 
But here we find known similarities regarding expected performance of this cheap power meter as it happens also with a cheap multimeter, it is to be expected high inaccuracy when you measure small electrical values.

When Kiriakos purchased his 6 1/2 benchtop DMM, the higher resolution of it along it accuracy, this changed everything.
This is a topic of great interest to me, regarding which one is the benchtop power meter of 2021, that we can trust and this still be reasonably priced?
 

[Kiriakos GR]

Indeed the standby measurement it does include PSU plus motherboard.
The motherboard this is configured to not power-up any USB devices when is off.
The only load could be an status indicator an Green LED.

It is a fact, that this measuring adventure, it did motivate me enough, so to research the market of 2021, and I did send few emails at brands (manufacturers) of benchtop single phase power meter, for product review samples.
It is too soon, so to say anything more.   

[Kiriakos GR]

I am very happy today, I did an accidental major discovery. 

At digital power meter calibration laboratory, Power factor limit regarding measuring error this is at +/_ 0.0004 PF
Test condition 220V 1A

This detail translates that at three digits PF meter resolution 0.000 PF , they might have double amount of error and this be undetected.   

[Andrew-88]

At digital power meter calibration laboratory, Power factor limit regarding measuring error this is at +/_ 0.0004 PF
Test condition 220V 1A

This an interesting information, it makes you wonder of how many digits of Power factor, we may trust down to 100mA or to 10mA.

[Kiriakos GR]

At digital power meter calibration laboratory, Power factor limit regarding measuring error this is at +/_ 0.0004 PF
Test condition 220V 1A

This an interesting information, it makes you wonder of how many digits of Power factor, we may trust down to 100mA or to 10mA.

I am now far more educated.

Bench top Digital power meter made at 2014,  auto-range changes to a lower range, if input this is at 30%.
Example: 10.000 mA range will drop to 5.000 mA range if input is 3.000 mA 

Dream machines of Digital power meter, they will use lower range (if there is one), when  input this is at 10%.