Now accurate are domestic electricity meters typically?

After a high electricity bill I've been trying to find out why. I've been checking my meter readings each day and so far (after 3 days) the usage is around 23kwh / day peak rate and 8kwh / day low rate (Economy 7). This seems very high to me. My house is a 3 bed semi, heated by gas (water also). Washing machine & dishwasher are always run at night in the E7 period. The only form of electrical heating is a 500w "frost watcher" in the [insulated] garage - I will fit my plug-in power watt / usage meter on this tonight to see how much it's on, but seems very little this time of year. No immersion heater. Most lights are compact flourescents, apart from ones which are on dimmers and a few low voltage fittings. (CFL's are in all the regulally used lights left on for long periods though). No electric shower or cooker. Oven *is* electric but used rarely, and only about 2kw IIRC.
There are 3 computers running 24/7, when I last checked the power used by these was about 60w each - will re-check. Will reduce this to 2 computers soon. Other stuff on 24/7 is a fish tank, again when I last checked total wattage was pretty low, and the heater doesn't come on much due to being in a warm room etc. Lights on for about 6 hours/day (fluroescents and Metal Halides). Will fit watt meter here after it's stint in the garage.
My equipment / power usuage hasn't really changed in the last few years, but looking at an electricity bill from 2001 the used units were way below what it is now.
I have an ammeter connected to a current transformer on the incomming mains supply, and even on a typical evening with lights on etc it reads about 4 amps, and during the day the meter is usually resting on it's end stop (0-80A scale).
So, I'm wondering of the accuracy of the electricity meter which was replaced about a year ago. Any tests I can do to confirm? I will have a better idea in about a week of power usage of equipment but to me I just can't see where a total of around 31 kwh per day is going!
Thanks in advance,
Alan.
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On Fri, 19 Nov 2004 11:16:55 GMT, Alan wrote:
<snip>

You don't say - is it a solid-state meter, or an old-style with a rotating disc? If it's disc type, they are moderately easy to check with a bit of simple arithmetic, the name plate will have a figure for the number of revolutions per kwh.
Switch *everything* off at each individual appliance - not just flipping off the consumer unit - and see if the disc is still revolving. If it is, then either there's something still on that you've forgotten about, or the meter is faulty.
If the meter stops with everything switched off then switch on one single appliance with a bit of load, like a 3kw fire for a few minutes, and take an *accurate* timing for, say, twenty revolutions of the disc. You should then be able to work out by simple arithmetic whether the meter is running fast or not.
Say the name plate gives 80 revs per kwh. You have 3kw of load, so in 60 minutes you'd expect 240 revs. Simple ratio will tell you how many kwh's your 20 revs represents.
If it's a modern solid-state meter, sorry, but you've got to rely on the company testing it.
--
the dot wanderer at tesco dot net

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Sorry, modern solid-state meter :-(
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am.ntlworld.com> writes

The principle is exactly the same, measure the exact time between 2 readings ie. start when the smallest digit clicks over & then stop again when the smallest digit ticks over some (much higher) reading later.
My electronic meter has a 0.01kWhr resolution, so the worst measurement reading error you can have if measuring over ~1kWhr is 1% and if you start & stop exactly when the dials (or lcd) clicks over then it will be better than that.
The load you are using (1kw fire or 3kw fan heater) will certainly not be an accurate load, just measured one here (1kW) and it was 1088W so 8.8% error. That will be the limiting factor of your accuracy.
So you should be able to check accuracy within about 10% quite easily which may suit you as it appears you are looking for gross errors rather than the last 0.1%.
--
fred

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Alan wrote:

Doesn't it have a flashing LED? Mine does - it gives 1000 flashes per kWh, so the power being used (in watts) can be calculated as 60 * flashes_per_minute or 3600 / seconds_per_flash.
AFAIK all domestic electricity meters are accuracy Class 2, which means 2% basic accuracy. AIUI this basic accuracy applies for a resistive load (unity power factor, no harmonics) for currents between the marked upper and lower figures (typically 20 - 80 A). For currents below the lower figure and for poor power factors (whether due to phase shift or waveform distortion) the accuracy is allowed to deteriorate somewhat.
--
Andy

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Remind me which way that is:)(.....
--
Tony Sayer


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tony sayer wrote:

Eh, what what is?
--
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I think Tony means to deteriorate in accuracy up or down.
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Accuracy.. over;( or under reading:)
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tony sayer wrote:

OIC what you mean: do they read high or low? I don't know really. You'd expect a rotating-disc meter to under-read at low power levels due to the friction in the bearings, but they have some sort of compensation for that, I think, with various tweaky-screws inside the meter. In principle power factor shouldn't make a difference because the accelerating torque on the disc at any moment is proportional to the instantaneous v*i product. If that's so then over a whole number of cycles the disc's movement will be proportional to the net energy flow - IOW the meter should respond to the true mean power. Errors, I guess, could then be in either direction. There are doubtless particular trends for actual practical meter designs, but I wouldn't like to speculate on that. Are there any meter specialists reading?
As to the electronic ones I have _absolutely_ no idea. How do they work? Do they sample and digitise v and i first and then compute energy flow in the digital domain - in which case accuracy will be down to the performance of the CT and VT and the ADCs - or do they use an analogue front-end with a 4-quadrant multiplier and integrator? - in which case the scope for errors would be rather greater...
--
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Andy Wade wrote:
How do they

    The last one's I worked on were operating AIRI, by using resistive current sampling, with an integrated amplifier and a/d convertor + a direct sample of V which was digitised. All the amplification and sampling took place in the same instrumentation, error corrected IC, with an external resistor for current and an external potential divider network for volts. The accuracy was very good, provided the current connections were sound. IIRC some units were trying out Hall effect current sensors, but the >10e5 current level variations could cause a few problems. The current input was of necessity integrated before amplifying and sampling, in order to cope with the problem of high speed transients from power factor corrected PSU's. The known problems were current resistor and semiconductor drift. The potential dividers were generally trouble free. Todays technology should undoubtedly be better than 15 years ago.
    Regards     Capitol
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Capitol wrote:
<stuff about electronic elctricity meters>
Interesting - thanks for that. Having mentioned CT and VT I realised, after posting, that that would be too expensive and that simple resistive coupling could be used, there being no need for isolation from the mains.

Which, presumably, just amounts to the anti-alias filtering you'd need in any A to D system. What sampling rate was used? - thinking about it if you're only trying to measure kWh (as opposed to kVARh) you're not interested in anything above the 50Hz fundamental - so the sampling rate could be very pedestrian...
--
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Andy Wade wrote: What sampling rate was used?
100KHZ at the time. See http://www.metering.com/archive/031/42_1.htm
for some present practice.
    Regards     Capitol
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Alan wrote:

Turn off the power at the CU, and see if the meter records any usage. Turn on the CU and turn off all circuits, and check that the meter still isn't recording usage. Turn on one circuit at a time. Etc - you get the idea.
Ben
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It certainly sounds buggered. Alternatively the chavs next door are stealing it, or there is something plugged in that you don't know about or forgot about, like a fan heater in the loft or something.
Christian.
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stealing
I see a weekend of testing coming up. Just found my power meter, blew the dust off and replaced the batteries. Ready for business now!
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Some models can be significantly inaccurate. I've measured 250% inaccuracy on some models of plug-in power meter for some loads.
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Alan wrote:

I'd recheck the PCs, unless they are old/low powered models, my guess would be that they are using a lot more than 60W each ;) My desktop uses between 150 and 300W depending on what it's doing. Whereas the laptop I use mostly now, is between 40 and 90W depending on whether I'm playing 3d games or not.
Also when my BiL had a 6ft marine tank it was costing him nearly UKP10/wk, lots and lots of low wattage stuff soon adds up when it comes to fish tanks - although the lights are usually the biggest load :) :)
You can roughly check the electronic meters by switching on a known load and counting the led flashes over a timed period.
Lee
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Lee wrote:

Oops, missed a "just" there, but you know what I meant :)
Lee
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It is a 6' marine tank, but it's been running 5 years and it's only recently my electricity bill has gone up. Most pumps on the tank are about 5w each, and only 4 of them in total. I'll check load though but don't expect it to be much.
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