Electric power consumption measuring device

Hello,
I run a number of computers at home, and was wondering how much electricity each one consumes. Is there some kind of device that can be attached between the PC and the wall outlet to measure power consumption?
Kind regards, Herminio Gonzalez
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On 26 Jan 2005 06:32:13 -0800, "Herminio Gonzalez"

About (finger in air) 200-500W (depending on the CPU spec, number of drives and monitor type) ;-)
Is there some kind of device that can be

Maplin do one ..
http://tinyurl.com/3l6hm
All the best ..
T i m
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Thank you T i m, that's just what I was looking for!
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It's likely to be significantly (double the real reading) inaccurate, the one I got from maplin is. It works fine on simple loads like motors/heaters.
In practice, you can look up all the figures on websites. 2-300W is unlikely.
I measured my server (12 drives spinning, 1.3G duron) at around 150W.
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That's discouraging. So according to your post, there's an error of a FACTOR OF 2? How could one know what the real power consumption is... Maybe I can calibrate it somehow, using devices with known power consumptions. But I can't think of any such devices...
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Unfortunately not. The meters (the cheap ones, the maplin and Lidl 6.99 ones) work out the current by measuring the voltage/current several times per cycle. Unfortunately, the lowest cost is got by reducing 'several' as much as possible.
For things like heaters, or motors, you can almost trivially get a pretty good reading of power and power-factor (amount current is leading or lagging voltage) with just 3 or 4 samples per cycle.
This is because the current they draw is a nice sine wave. Many PC (and other switched-mode PSUs) unfortunately have significant very fast variations in current. So, the meter misses these changes, and assumes that the power supply is drawing a higher current than it should.
It's basically a similar problem to trying to join up the dots. If you've got lots of dots, then you can make a nice smooth curve, without worrying about the shape of the curve you make being different from the intended one. As you reduce the number of dots, eventually you get to a stage when you're guessing.
More expensive meters should do better, unfortunately, determining how much goes into the electronics is hard.
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I bought one of the current Maplin ones, and I think that might not be too bad. I'm having trouble getting my proper true power meter back from someone I've lent it to (my boss;-), so I haven't been able to do a real check on the Maplin one.
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).

My proper true power meter actually uses an analogue multiplier and integrator, so it has more than enough dots so there's no need to guess how the line goes between them ;-)
--
Andrew Gabriel

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What's the name, I've forgotten the name of mine, IIRC began with a b, and was german.

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

True. It's hard to tell how much someone knows on usenet though.
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Yes, that's the crap one which is way off -- I think the model was PM230. The one Maplin stock now is a different make and I can't remember what it is (nothing well known though, but it is Asian not German).

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.
--
Andrew Gabriel

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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

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snipped-for-privacy@isbd.co.uk wrote:

<snip>
Even the cheap ones do power factor. I'm fairly confident that most of these devices have a single channel 8 bit A/D (minimum current is typically 40mA = 13000mA/256), sampling at 400Hz or so. (or maybe 50Hz, with undersampling, I'm not sure.)
Add a 4 bit micro, and an LCD, and you've got a power meter.
A 0 power factor device can have a very spiky current draw. My PC power supply draw looks like a sharks fin, with a very sharp rise, and a slow decay to a fast fall.
Typical older SMPSs with a bridge rectifier feeding a capacitor (which read correct) have a current draw that looks like the top of a sine wave.
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In uk.d-i-y, Ian Stirling wrote:

Presumably if you measured total energy consumption over a long period, things would even themselves out. But how long, I wonder? My Maplin meter stops at 9999 kWh.
--
Mike Barnes

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No, they don't. It consistently reads high by the same amount for hours or days.
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But *how* do you measure "total energy consumption over a long period", that's the original problem! If the meter's view of instantaneous power consumption says 1kW thenit's going to consume (according to the meter) 9999kWh in 9999 hours (unless the instantaneous reading changes of course).
--
Chris Green

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In uk.d-i-y, wrote:

If the draw looks like a sharks fin, then the instantaneous reading *will* vary won't it? Unless the sample times are synchronised to the wave form.
Never mind. I'm quite happy to believe that the readings are unreliable for PC power supplies etc.
--
Mike Barnes

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<snip>
With my observations, it diddn't, but was consistent within a range of voltages (about +-2%) and with a 1.8 ohm resistor wired into the mains lead.

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     snipped-for-privacy@isbd.co.uk writes:

If it did this, it would under estimate rather than 3 times over estimating. That's why I said the form of the error was puzzling.

Yes, although if you don't sample fast enough to see the general form of a rapidly changing value, you will get a completely misleading impression (lookup Nyquist's Theorem).
--
Andrew Gabriel

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No it wouldn't. It could have lots of amps flowing at 240 volts but with (say) 10% power factor the Volts*Amps would be way more than the power.

I was thinking of 'averaging' as in a moving coil meter and rectifier, that doesn't sample in the sense you mean at all, it genuinely averages the current (or voltage).
--
Chris Green

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     snipped-for-privacy@isbd.co.uk writes:

The example was a SMPSU, not a capacitive or inductive load. Averaging a SMPSU current pulse or failing to take into account it only happens when mains voltage is around 340V would give an under reading. I can't imagine what mistaken assumption they've made which results in such a load 3 times over reading.
--
Andrew Gabriel

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<snip>

To quote from an earlier post I made on this. "
A quick patch involving a 1.8 ohm resistor in the neutral lead and resistive dividers protected by zeners indicates that the current has a really odd shape.
The couple of switchers I checked exhibited the expected short half-sine looking pulses at maximum voltage. The power indicated by the meter was within 20% of my eyeballing the scope.
On trying it on my PC, I found a rather odder shape sort of sharks-fin like shape. The current rises to a peak of 2.2A in .4ms, then decays over the next couple of milliseconds more or less linearly to 0.6A, then drops in 0.5ms to 0.
That's a total of 2.25Ams per half-cycle, at 320V, that's .721Ws per half-cycle, or 72W when idle.
The meter however reads 138W.
And in suspend, it's around 30W, around 10W 'off'.
"
It may well be assuming that the current will be either a 'normal' sine wave, with a power factor of whatever or a sharp chopped-sine pulse near the wave peak.
This worked fine, until various regulations about harmonic content on power supplies came in. Then the PSU makers started working out the cheapest way to meet these directives. This ended up as very odd waveshapes.
As to why. Well, let's assume that it samples at the peak of this pulse, 2.2A.
Let's try 2.2A * 2ms = 4.4mAs. 4.4mAs * 320V = 1.4J * 100 cycles per second = 140J/s, or 140W.
Surprisingly this works quite well.
However, I suspect the real answer is that it's trying to make a nice smooth wave out of it. To get an accurate number, you're going to need to sample at better than 1Khz.
Alternatively, if it's sampling at 500Hz or so, and trying to assume that the waveshapes may be symmetrical (be it intentionally, or through filters) then it may get vastly misleading answers.
Especially as we're not talking about a 2000 quid scope that's had hundreds of man-years spent on the design, and verifying it's correct, but maybe a few hundred hours and checking on a few loads around the office.
Anyway, hypothesising on how it fails is kind of pointless, in the face of the fact that it can't be repaired (practically)
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