Earth Notes: On Plug-in Mains Power Meters for the UK: Review (2008)

Review: N67HH vs 2000MU-UK (L61AQ) vs PM230

(AKA "power monitors" or "energy monitors" or power vampire finders!)

Recommended: N67HH, see below...

PM230

At the time the quickest and easiest and cheapest for me to buy (on-line) was the PM230 kWh meter from EnergyOptimizersDirect. It worked well and helped in identifying what we'd need to work on first to trim our electricity consumption. Especially my office computer network and our domestic fridge/freezer. Both have now been replaced to good effect.

Just being able to plug an appliance in, and plug this unit back into the original socket is very easy, and the unit is robust and easy to read. I have never bothered putting in a battery to retain readings and set-up.

A weakness of the PM230 is that is seems not to be able to measure below ~7W. That limits its ability to monitor already-efficient appliances.

The PM230 is also a little fiddly in requiring you to page it through various modes. It requires several pushes to get to the 'W' reading that I use most. That can be tricky when the meter/socket is difficult to access.

Possibly even more annoying is that given the layout of the PM230 the appliance lead from the plug partially obscures the display. That can make the central button difficult or impossible to use.

2000MU-UK

As of 2008-03 I was sent a 2000MU-UK meter stocked by Maplin as L61AQ for as little as £10 during special offers. (I was sent mine by someone who had two!)

Three things struck me immediately on using the 2000MU-UK:

1. It was a little quicker and easier to use with a single button push for the Watts readout.
2. It can report much lower power readings, somewhat reliably, down to 1W increments but with a ~3W error (eg it can show 3W with nothing plugged in).
3. It tends to give lower readings for many appliances, possibly because it can measure lower-power items more accurately. It also may be correcting better for power-factor (the difference between VA and W). That is important for electronic gadgets.

Although the 2000MU-UK seemed robust enough to me, I had reports that it can be fragile in some circumstances. So I'd advise against heavy-handed jamming-in of plugs, etc.

I don't have a test rig to measure these meters' accuracy against. The fact that the 2000MU reads consistently lower than the PM230 does not indicate which is the more accurate though the 2000MU seems more precise (ie able to measure smaller values). But I'm inclined to believe the 2000MU values since they accord with other independent measurements that I have been able to make.

The 2000MU-UK is as robust and easy to read as the PM230. So given its sometime lower price and easy availability from Maplin, and its possibly better accuracy and precision, I prefer the 2000MU-UK over the PM230. But the N67HH below trounces both in my opinion!

November 2009: The Maplin N67HH

(Also labelled KES02-01 on the device, Nikkai brand.)

2009-11-17 I saw this on special (price down again 2009-12-16 to £7.99) in my local Maplin branch, and it offers some interesting features:

• Claimed resolution down to 0.1W. (The other meters don't get close to <1W in practice which makes accurate measurement of consumption of low-power devices essentially impossible.)
• A claimed self-consumption of < 0.5W. (Other may meters consume significantly more, eg 1.4W for the MU2000-UK though the PM230 only draws ~0.5W, which is a bit of a waste if measuring for a while.)
• Horizontal arrangement which may help in some tight spaces.
• Seems to be able to capture peak power.
• Has internal (rechargeable) batteries. (May help retain running tallies in some circumstances, etc.)

A minor worry is that various parts of the labelling and instructions claim that this should not be used with loads above 10A, 13A and 16A. This should be clarified, but I think that I can assume that 13A is safe, though there might nominally be some loss of accuracy.

It also seems difficult/impossible to reset the cumulative kWh record without completely resetting the unit. And the kWh figure is only shown to one decimal (eg 0.1kWh), but for my purposes that is only an annoyance.

Note: "EMA-1" wrote to me 2010-06-29 with the following:

You can reset the Kwh counter alone by using FUNC to display the accumulated kwh hours counter, and then holding the FUNC button for five seconds or until the counter is reset.

A few months ago I began creating instructions for this type of meter after searching the web for instructions (for myself) and finding that many people had difficulty operating the meter and getting the most from it.

I created updated instructions, minimum instructions, and quick instructions to offer a few different ways of getting the job done.

It turns out there are many similar meters and therefore many which use these instructions.

I immediately tested this out on the lowest real mains load that I have to hand, our children's plug-in LED night-lights rated on the back at 0.5W. The meter showed 0.6W consumption which is within claimed (0.1W) accuracy. (Interestingly the reported consumption did not drop when I increased ambient light level and the night-lights automatically turned off.)

I also tested a reading lamp to make sure that its 'off' switch really is off and according to the meter it was (some can still consume standby power).

I also re-tested our baby monitor; each end seems to consume under 2W idle and a little over 2W when a link is established. Better than I had anticipated.

Measuring consumption of my SheevaPlug showed:

• At start-up and with a heavy load on the USB flash drive: 7.3W.
• With the USB load stopped (and suspended) but still flat-out CPU: 6.0W.
• With the system quietening down after ~15m: 3.5W spiking to ~6W.

This is giving me greater precision than my in-line DC power meter!

I have also now used this to measure consumption of our new dishwasher and it seems to cope with the higher (2kW+) loads fine too.

I think that for me this is the best power meter by far, so I suggest that you rush out and buy one!