In article ,
Ian Stirling writes:
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.

I bought one of the current Maplin ones, and I think that might
not be too bad. I'm having trouble getting my proper true power
meter back from someone I've lent it to (my boss;-), so I haven't
been able to do a real check on the Maplin one.

I bought one of the older ones Maplin stocked a few years ago,
and that's hopeless on non-resistive loads (out by a factor of
3 on SMPSU's -- too high, which is a puzzling way for it to be
in error).

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.

My proper true power meter actually uses an analogue multiplier
and integrator, so it has more than enough dots so there's no need
to guess how the line goes between them ;-)

--
Andrew Gabriel