Aniseed wrote:
That's discouraging. So according to your post, there's an error of a
FACTOR OF 2? How could one know what the real power consumption is...
Maybe I can calibrate it somehow, using devices with known power
consumptions. But I can't think of any such devices...

Unfortunately not.
The meters (the cheap ones, the maplin and Lidl 6.99 ones) work out the
current by measuring the voltage/current several times per cycle.
Unfortunately, the lowest cost is got by reducing 'several' as much as
possible.

For things like heaters, or motors, you can almost trivially get a pretty
good reading of power and power-factor (amount current is leading or
lagging voltage) with just 3 or 4 samples per cycle.

This is because the current they draw is a nice sine wave.
Many PC (and other switched-mode PSUs) unfortunately have significant
very fast variations in current.
So, the meter misses these changes, and assumes that the power supply is
drawing a higher current than it should.

It's basically a similar problem to trying to join up the dots.
If you've got lots of dots, then you can make a nice smooth curve, without
worrying about the shape of the curve you make being different from the
intended one.
As you reduce the number of dots, eventually you get to a stage when
you're guessing.

More expensive meters should do better, unfortunately, determining how much
goes into the electronics is hard.