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 Post subject: Do voltmeters give accurate information?
PostPosted: Thu Aug 04, 2011 3:39 pm 
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Hello

I'd like to check how much power my work PC uses, and am thinking of getting a voltmeter.

However, I was told that voltmeters aren't reliable with PCs because their PSU is more complex than basic power supplies like those for a fridge.

I don't have the technical knowledge to tell: For those of you who do, are voltmeters indeed unreliable to estimate the power used by a PC, in which case, is there an application that will list all the components in the computer and compute its power requirements?

Thank you.


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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Thu Aug 04, 2011 4:42 pm 
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A voltmeter will not help you. Go instead for a Kill-a-Watt or similar (like Seasonic Power Angel). These measure AC power at the socket -- the electrical appliance plugs into it & provides a range of info, including real power (not just VA). KaW is really quite cheap now, I think I've seen them for as little as <$20.

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Fri Aug 05, 2011 6:11 am 
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Hi, I have a cheapy (off eBay) power meter job and when PC is off / sleep I'm fairly sure it's wrong! the load values, gaming ~200w and Prime + FurMark >300w I can believe but when PC off or asleep it reports ~40w. If I flick the switch on the back of my Antec Signature PSU it then reads 0w.
Thinking about 40w light bulb for heat and then PC with no fans running and nothing with any noticable warmth I simply don't believe this figure and think it's some interaction between the meter and the PSU. My 5.1 Yamaha amp also reports considerable (20~30w) just from being on too, not I believe that either.

Seb

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Fri Aug 05, 2011 6:59 am 
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littlebigman -- I just noticed that the article you linked actually refers to a Kill-a-Watt as a Voltmeter, which is a total misnomer. It is a power meter. Voltmeter usually refers to a multimeter -- http://en.wikipedia.org/wiki/Multimeter -- which is not useful for power measurements.

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Fri Aug 05, 2011 7:11 pm 
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SebRad wrote:
My 5.1 Yamaha amp also reports considerable (20~30w) just from being on too, not I believe that either.Seb


Slightly off topic, but:

If it has Class A/B amplification circuitry, and doesn't have a sleep mode, then 20-30W is typical - it trades off high linearity for efficiency. Typical for old style amps with behemoth power transformers. Newer amps that use Class D circuitry can idle at a few watts..but may have less than stellar linearity.

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Sat Aug 06, 2011 6:07 am 
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MikeC wrote:
littlebigman -- I just noticed that the article you linked actually refers to a Kill-a-Watt as a Voltmeter, which is a total misnomer. It is a power meter. Voltmeter usually refers to a multimeter -- http://en.wikipedia.org/wiki/Multimeter -- which is not useful for power measurements.


Another oddity is that they recommend using an inkjet by turning off the power to it on a regular basis. Many inkjets will leave the cartridge open and the ink will evaporate if you turn the power off. Way back in the day my dad printed about 10 pages a year with an inkjet and he had to buy a new ink cartridge every 3 to 6 months because he was turning the printer off by way of a power strip instead of waiting for the extremely long process of shutting the inkjet down by pressing the power button and waiting for it to seal up the cartridge (it was a long and noisy procedure).

And that isn't going into the issues with color inkjet alignment/prime routines that waste large amounts of ink. Or smart cartridges that say they are out when there is still ink left (based on date of manufacture or power on hours instead of just monitoring the actual ink level).

It's widely known that printer ink is way overpriced:

http://www.cockeyed.com/science/gallon/liquid.html
http://arstechnica.com/hardware/news/20 ... prices.ars
http://www.theregister.co.uk/2003/07/04 ... imes_more/

and on and on...

It's poor economics to save a few dollars a year on electricity to end up having to pay several times more dollars for ink.

My father now has a laser printer and turns it off between uses just like he did with the inkjet. He now gets several thousand pages per toner cartridge vs the 1-10 pages per inkjet cartridge. He went through the 1000 page starter cartridge in the first 5 years so I guess he's up to a couple of hundred pages a year now. The current cartridge may outlast him.

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Sat Aug 06, 2011 6:45 am 
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littlebigman wrote:
Hello

I'd like to check how much power my work PC uses, and am thinking of getting a voltmeter.

However, I was told that voltmeters aren't reliable with PCs because their PSU is more complex than basic power supplies like those for a fridge.

I don't have the technical knowledge to tell: For those of you who do, are voltmeters indeed unreliable to estimate the power used by a PC, in which case, is there an application that will list all the components in the computer and compute its power requirements?

Thank you.


I've seen videos on youtube (linked from jonnyguru.com) showing vast inaccuracies at high wattages (think 500+). But I haven't see anybody say they were inaccurate with lower loads like the typical home user PC that idles under 100 watts.

http://www.jonnyguru.com/forums/showthread.php?t=2695 (varying wattages, killawatt reasonably accurate)
http://www.jonnyguru.com/forums/showthread.php?t=3019 (1200 watt PSU, killawatt inaccurate)
http://www.jonnyguru.com/modules.php?na ... 2&reid=191 (1000 watt review accurate)
http://www.jonnyguru.com/forums/showthr ... 284&page=3 (general discussion)

The general consensus is that it'll likely be within 2% under 500W and you'll have to use some common sense or a second tool if you are serious about higher wattages. For home users it's fine. Just use it and trust it.

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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Sat Aug 06, 2011 10:45 pm 
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Posts: 40
This one is actually fairly easy to answer*.

Power is measured by multiplying volts and amps. As you might know, current and voltage on the mains changes constantly; it is a (by approximation) sinusoidal waveform with a frequency of 50 or 60 Hz. Apart from variability in how much power your device actually draws, it is not possible to at any one time take the voltage and current value and multiply them, because they are in a state of constant flux.

There is two ways to attack this problem: (1) you measure current and voltage many, many times per second, multiply them together and multiply them by the time it takes for each sample or (2) you measure it fairly slowly and kind of guess how the voltage and current have behaved in the meantime. The first one, when executed properly, is called true rms measurement while the second doesn't necessarily have a name.

You want a true rms power meter. Why? Especially at low power draws, computers and other equipment produce unpredictable, not-like-a-sine-wave-at-all current waveforms. This is called a low power factor. The assumptions that non-true rms meters make to estimate power draw fall flat on their face in these instances. The kill-a-watt power meter is actually true rms and you american people are lucky to have such a good brand-name, relatively cheap true rms power meter widely available. In most other parts of the world it is hard to find out if a power meter is actually any good.

That is not to say that you should believe every digit of your readings. Most if not all consumer quality power meters have at best 1% accurate voltage and power measurement, but more importantly, they have a specified offset error of at least 1W, but often 2W. Or they don't specify worst-case error at all and just post typical accuracy. This means that if you are measuring an especially low-power PC that for instance draws 15W, the margin of error is 1% (0.15W) plus that +/-1W offset, which is about +/-6.5%. And, but I'm really digressing here, this is often a biiiiiiiig misunderstanding that leads to inaccurate power supply efficiency measurements. Review sites may think that the 1% specification says everything about the accuracy of their measurement equipment, and they post results that for instance a picopsu+adapter combination is 85% efficient at 15W output. Well, you cannot know it that exactly, because your margin of error is probably about 6%, so it can be anything from 79 to 91% efficient. [/rant]

Using a multimeter for power measurement is useless. They can only measure one quantity at a time, either voltage or current. They cannot multiply them together, unless you get into the really nice stuff like Gossen Metrahit Energy, which will set you back $1000. And although I say that consumer power measurement devices are not terribly accurate, getting better accuracy means the same thing for your wallet: proper power analyzers with specified 0.1-0.25% accuracy are hard to get below a grand. That is why consumer-level devices are really the only way to go, and the kill-a-watt is an excellent buy in that category.

*he says at the start of a tl;dr post


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 Post subject: Re: Do voltmeters give accurate information?
PostPosted: Mon Aug 08, 2011 1:31 pm 
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Joined: Tue Dec 11, 2007 8:54 pm
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MikeC wrote:
A voltmeter will not help you. Go instead for a Kill-a-Watt or similar (like Seasonic Power Angel)


Thanks everyone for the feedback. I bought an Avidsen 103353 power meter similar to a KaW. I'll connect it to my PC and see what it reports.


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