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Customer Review

61 of 64 people found the following review helpful
2.0 out of 5 stars Take readings with a bucket of salt, 24 Nov. 2009
I got a couple of these devices, one for work, and one for home.

At work, it was reading 1.3kW. I have a plug-in power meter (which measures amps AND volts, unlike the Efergy which just measures amps) in our server room which was measuring 670W or 780VA. This feeds into a UPS which was reading a load of 3A (so about 750VA - similar to the plugin-in meter). When I unplugged the UPS supply, the Efergy dropped to about 1kW - so it only measured a drop of 300W, not the 600-700W it should have done. That's a big difference, relatively speaking.

FWIW - our annual electricity bill suggests we use an average of about 2kW all the time (which is why we are seeing if we can monitor it better), so during the daytime, it looks like the Efergy was reading quite low - maybe the 1kW was right, and the 1.3kW was wrong - you can't easily tell.

Then I tried the other Efergy which was destined for home, which consistently gave readings 200 watts below the first one!

Just to add even more fun, we had great confusion because when we turned OFF some fluorescent lights (about 80W worth), the reading went UP (by 300W, not an insignificant amount!), and when we turned them ON, the reading went DOWN - so according to that we should leave the lights on all the time! (As far as I can think, this may be due to capacitive and inductive loads cancelling out when the lights are on - however it makes the devices hard to use for what they're intended for).

So, while these things are easy to install, and can give a rough idea of power usage, don't take them too seriously, and be especially careful if you have fluorescent lights or lots of computers.

We may need to look around for a smart meter which can take power factors into consideration, even though it will need an electrician to wire it in.
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Tracked by 4 customers

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Showing 1-6 of 6 posts in this discussion
Initial post: 3 Sep 2010 15:27:47 BDT
D. Hughes says:
Very good - it explains the technical stuff easily enough for non-technical people, like me, to understand.

In reply to an earlier post on 29 Sep 2014 22:27:13 BDT
Adam Drake says:
Very interesting, thanks. I will only add that people need to remember that the Watt (W) is a unit of the amount of work done. There is a certain amount of work being done just by moving all the electrical currents around the home/office. Certain devices (the fluorescent tubes as mentioned earlier) can act as bottlenecks to the flow of electricity and make it harder for the current to flow.

When the smart meter reads a small amount of Watts even when nothing is turned on or in standby, this is the result of what was mentioned above, compounded by the slight inaccuracy of the device. For a normal sized home, work with the assumption that the the smart meter is only accurate to about 90% and you'll get good data on how much money you are spending.

Posted on 11 Oct 2014 15:13:54 BDT
Beryl says:
Thanks P.D.Smith, this confirms part of my experience with these devices. 5 years ago I was 'promoting' the Watson and Holmes as a way for residents to reduce electricity bills. But despite them being more expensive than most others such as Eco Eye, there were still significant inconsistencies (inaccuracies) with the measured bills by the utility company - sometimes more than 33% out, which didn't go down well with the users.

I was hoping that 5 years on, we would have removed most of the inaccuracies to a tolerance of acceptability; and setting up and interpreting would be easier. P.D.'s comments seem to indicate there's a still a ways to go before the average Joe (like me) can rely on these things, without bringing in a qualified electrician (to fit and calibrate) who will charge 3 times the cost of a more complicated device!

I suggest these meters are useful, can help economise and are valuable if you can compute the real impacts of
power factors, fluorescent lights and other long running devices with 'sleep' settings. It seems to me that these devices still require more 'clever design' paired with simplicity at the human interface. Computers do this, I'm sure dedicated micro processors can do it by now!

Posted on 29 Dec 2014 21:28:52 GMT
MRB says:
Interesting about the fluorescent lights! So I tried this out on various fluorescent lights in my home, but my results were totally different from yours. When the florescent lights were turned on the the consumption always went up (in my case) and when switched off consumption goes down as results from the meter. Only problem is that the amount consumption goes up as reported is exaggerated, as the Efergry reports about double of what the fluorescent lights should be consuming. However, I am still very enthusiastic of this product as it serves my needs as expected. Having helpers with my mother-in-law, it helps me control whether they are keeping electric radiatiors throughout the whole night when they are asleep.

In reply to an earlier post on 8 Mar 2015 22:59:22 GMT
Last edited by the author on 9 Mar 2015 10:02:55 GMT
Geoff says:

This seems to be a rather dubious explanation about electrical matters.

The likely reason why the current flow (all this device measures) increases when the fluorescent lights were turned off is because they possibly have a reactive component of current which is leading (possibly due to capacitive ballast) which offsets some of the of lagging reactive current in the system (most likely due to the induction of electric motors on the same circuit).

For example, at 247V, 50Hz AC, a desk fan of 17.4W take a current of 111.9mA, to give 27.6VA, with a power factor of 0.63. An LED bulb of 6.3W, takes a current of 72mA, to give 17.7VA, with a power factor of 0.36. As expected, together they use 23.7W, the sum of the individual powers, but the combined current is only 104.8mA at a power factor of 0.92, less current than the 111.9mA for the fan alone, with a combined 25.9VA, which is less than the 27.6VA for the fan alone. The sum of the two values would be 45.3VA This show the need to measure and take account of both the voltage and the power factor. Simply measuring the current alone cannot give a reliable result.

This is a well known effect in electrical systems and is not sensibly explained in terms of the "fluorescent lights acting as a bottleneck", which they do not, as they are not in series with the rest of the system.

Unless both the voltage and the current are measured and compared, both in magnitude and phase, such devices cannot give a reliable measurement of power.

I have recently been calibrating a plug in "Energy Consumption Cost Meter", using a digital oscilloscope with maths capability, and with a 6W LED bulb this meter gave three time the power reading, about 18W, even though the meter was supposed to measure the power and power factor, but always gave a power factor of 1.0, even though in this case the power factor was about 0.33, and the power reading was actually the Volt Amps.

Be very circumspect about the readings produced by these energy meters, as they can be considerably in error.

In reply to an earlier post on 8 May 2015 08:45:33 BDT

"Unless both the voltage and the current are measured and compared, both in magnitude and phase, such devices cannot give a reliable measurement of power."

Exactly so. And many of these types of meters only measure voltage and current without regard to the phase and therefore give a reading in VA rather than Watts, with significant error when the power factor is less than unity, as it often is with modern devices using switch-mode power supplies.

And Adam Drake, the Watt is a unit of power, not of energy, so does not represent work done. Work done is power and time, hence Watt-Hours. 3600 Watts for 1 second is the same amount of work as 1 Watt for one hour. They are massively different power levels, but the same amount of energy.
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