Andrew Gabriel wrote:

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).

If it does not sample fast enough, then it may well overestimate
current consumption.

Yes, I guess if the spike width at the voltage peaks is small
compared with the sampling rate (hello Mr Nyquist;-) and the
sampling is somehow always catching it (prehaps due to synching
with the voltage waveform), then that would account for it.

Isn't it as much to with low power factor as anything. The
cheap/simple power meters simply measure average (and I mean average,
not RMS) voltage and current, then multiply the two together and apply
the right factor to convert to RMS and say the product is power. It
matters not one whit how fast your samples are if you do this, the
answer will always be wrong.

--
Chris Green