John Rumm wrote:

[snip 7 quid power meter from Aldi]

There is a secondary issue here as well, if the description is to be
believed then this one does not seem to take account of the power factor
of the connected appliance either. Hence on reactive loads (computer
kit, fridges, freezers, and the posher power tools) you will get answers
that are somewhat out.

Aldi still had these on offer this week in Kendal. It claimed on the pack
that it displayed power factor so I bought one to "play" with. It certainly
does more than they say in the brief advert but I'm not so sure how
accurate it is for small loads. FWIW here's my opinions after giving it a
bit of use.

It displays the power factor and the power (both instantaneous watts and
cumulative KWh), plus voltage, current and frequency. For those who don't
want to do simple arithmetic it can be programmed with the price per KWH
and displays the total cost along with elapsed time "on load". A neat
little extra is that you can set up 2 different prices for different
periods of the day which may be of use for those on economy 7 tariff.

It also indicates maximum current and power, which AFAICS can only be reset
by removing the batteries (2 off AG-13 button cells).

There's a facility to set an arbitrary "overload" power level which causes a
red LED to light up if it's exceeded, I can't see any real use for this but
I suppose it might appeal to some. Nor can I see the point of displaying
the frequency, I expect the mains frequency is regulated to much closer
tolerances than the accuracy of this meter.

The claimed accuracy leaves something to be desired. Voltage and current are
+/-3% but power is +/-5% of displayed value and/or +/-10 watts so not much
use for checking the low power consumption of things like TV's on standby.
I checked it against known resistive loads like tungsten lamps and 3KW
heaters with reasonable results, it displayed the right values for a 20w
halogen lamp and an 11W CFL but a 7W CFL came up as zero watts! Inductive
loads like the freezer and dishwasher motor were quite erratic, in
particular the value for the dishwasher swung erratically from about 120W
down to near zero with corresponding power factor swinging between 0.9 and
near zero.

Would I recommend one? As a serious measurement tool - no, but as a cheap
and cheerful indicator it's probably OK. It's main use for me was to assess
the overall consumption of things with a variable duty cycle like the
freezer, or the washing machine and dishwasher where it's hard to estimate
just how long the heater is no for each cycle. For most other devices just
looking at the rating plate or counting the revolutions or flashes on
the 'leccy board meter is probably the best approach.

--
Mike Clarke