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