[conspire] How accurate, etc.: AC Power (Watt)meter (What's Your Watt?/...)

[Tony Godshall]

https://en.wikipedia.org/wiki/Sleep_mode

generally, in "sleep mode" the RAM is still powered
so it can resume state, whereas in "hibernate" RAM's
state is stored to disk for a full power off.


On Wed, Apr 22, 2015 at 9:11 AM, Paul Zander <paulz at ieee.org> wrote:
> Michael,
>
> Thank you for the very good explanation of the subtleties of AC power.
>
> But the manual does indeed have a section on specifications:
>
>     Specifications
>         5 ft. cord connecting display and socket
>         Electrical Rating: 120V~/15A/60Hz/1800W
>         Maximum Watts Displayed: 1800
>         Maximum Dollars Displayed: 9999
>
> They don't state _anything_ about measurement accuracy. As someone who has
> had to read the many pages of detailed specs for an oscilloscopes and
> voltmeters, it makes me wonder what they are hiding.
>
> The reality is that most of our homes consume a fair amount of power even
> when everyone is sleeping.  Obviously the refrigerator needs to cycle.  But
> the microwave and coffee maker consume a constant trickle just to keep their
> clocks on.  Then there are the assorted networking routers, etc.   On
> occasion, my wife has asked if I have turned off the computer, because
> several LEDs are always on, unless I turn off the wall outlet.
>
> Actually I wonder what is the difference in power between "OFF" and "SLEEP"?
> The monitor screen is black.  The disk drives are off.  The only visible
> difference is the "power" light which is off when "OFF" and slowly blinking
> in "SLEEP".
>
>
> ________________________________
> From: Michael Paoli <Michael.Paoli at cal.berkeley.edu>
> To: conspire at linuxmafia.com
> Sent: Sunday, April 12, 2015 10:21 AM
> Subject: [conspire] How accurate, etc.: AC Power (Watt)meter (What's Your
> Watt?/...)
>
> Well ... it varies.  ;-)
>
> But yes, good points - not so much that "they" or "it" is necessarily
> inaccurate or that inaccurate, but good to generally be aware of how
> (in)accurate one's measurements are.
>
> Most test instruments typically have specifications, that well spell out
> their accuracy.  And yes, the specifications generally are quite
> accurate - and especially from reputable manufacturing companies.  That
> doesn't mean the specifications are always 100% correct, but they're
> generally intended to be correct, and for the most part are.
>
> That being said, some related bits ...
>
> First, I was curious on the "What's Your Watt" - specifications?  How
> accurate *is* it?
> A trace of sleuthing and I found ...
> http://www.sfenvironment.org/whatsyourwatt
> http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf
> ... apparently Belkin Conserve Insight
> ... as far as I can tell, seems they have, for USA(/Canada) power
> compatible versions, 3 very slightly different versions of this device:
> Part # F7C005
> http://www.belkin.com/us/support-product?pid=01t80000002zwZFAAY
> Part # F7C005q
> http://www.belkin.com/us/support-product?pid=01t80000002HrhcAAC
> Part # F7C005fc
> http://www.belkin.com/us/support-product?pid=01t80000002If1gAAC
> But I also notice, all 3 have the exact same User Manual:
> Conserve Insight F7C005 v1 - User Manual (US) (English)
> http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf
> which is exact same URL also found further above.
> In all the checks, I didn't find specifications that covered the
> accuracy of its power measurements! - somewhat surprising.  But it does
> give wee bit about it its inaccuracy at (very) low power levels:
> "
> Your Conserve Insight uses sensitive
> electronics to measure the amount of power
> flowing to your connected device. When
> the connected device is consuming a very
> small amount of electricity, it becomes
> very difficult to distinguish between the
> power being consumed and the electrical
> noise found at all power outlets. To ensure
> accurate readings, the Conserve Insight
> stops showing values below 0.5 watts and
> displays the '0-0.5 Watt' screen.
> "
>
> Anyway, notwithstanding the lack of more detailed specification (it does
> gives specifications, but alas, doesn't state how accurate the power
> measurements are), it is from a reputable instrument manufacturer, so
> it's probably *fairly* accurate ... at least compared roughly to
> comparable test instruments (e.g. electronic measurement instrument in
> roughly the $15 to $50 USD price range ... and in probably about 2012 or
> so).  So, a good guestimate might be something in the range of roughly
> +-2% to +-5%, or so.  Since Belkin also quite implies it can't
> accurately measure <~=0.5 Watts (W), one might guestimate accuracy of
> something like the worse of:
> +-2% to +-5%, or 0.5W, whichever is greater.
>
> But more on how (in)accurate.  Measuring Watts isn't as simple as
> measuring Volt-Amperes(Amps) (VA).  Watts - or true power consumption,
> is the *instantaneous* product of Volts and Amps - and since that
> continuously fluctuates for Alternating Current (AC), that's generally
> given averaged over some reasonable interval (one or more full cycles,
> e.g. for 60 Hertz ((Hz) Cycles Per seconds - CPS), one second would
> cover 60 cycles).  The difference between VA and Watts, is VA is mere
> product of the Root Mean Square (RMS) values for each of V and A.  Note
> that VA and Watts can give *very* different numbers.  W is the true
> power consumption, whereas VA is not - though knowing or also knowing
> VA can be useful in certain contexts, and sometimes/commonly, VA may be
> a rough, to even good, approximation of W (and knowing the VA is
> generally better than having no W information).  Without explaining RMS
> in detail, think of it as "equivalent".  If one has a pure resistive
> load, let's say of 1 Ohm, applies a Direct Current (DC) Voltage of 1 V,
> then one has current of 1A, and power consumption of 1W.  If instead of
> DC, and AC voltage is applied, regardless of what waveshape applied, if
> the AC RMS voltage is 1V, the RMS current will be 1A, and the power
> will be 1W (at least averaged over any full cycle).  Note also here,
> talking theoretical resistor (same resistance regardless of
> current/voltage).  Anyway, that's a key reason why RMS values are so
> useful - for their relative equivalence.  Better electrical instruments
> will give true RMS values, other less accurate ones will give an
> estimate, typically taking some other measurement and presuming the
> waveshape is sinusoidal (most commonly it's fairly close to sinusoidal
> - particularly for common power situations, but that's not always the
> case), and will then give a reading based upon that presumption.
>
> Anyway, W vs. VA ... phase matters.  The more out-of-phase they are,
> the more they'll differ.  Bit of crude analogy - think of pushing
> someone on a swing - regularly push as they start the downward portion
> - that's in phase - adding energy to the system - to build their
> swinging - or at least offset losses from various friction and such.
> If instead, one pushes as they're approaching towards the top of their
> swinging motion towards you, that's out-of-phase, and instead of adding
> energy to the system, is taking energy out, and reducing their
> swinging.  One might be doing the same amount of pushing in either
> case, but not only how efficiently energy is flowing - but even which
> direction - depends on the phase relationship.  Such is the case with
> AC.  Measuring one's pushing, while ignoring phase, would be akin to
> measuring VA (or at least one of its components), rather than properly
> measuring and determining W.
>
> Ye olde power company electric meters.  I forget the precise details of
> how they do it (I read the description many decades ago), but they very
> much measure W, not VA.  They very effectively and quite efficiently,
> do a continuous "instantaneous" electromechanical multiplication of the
> V and A - and thus measure true W.  Well, ... *almost* instantaneous.
> The electrical field parts of it they use to do that are ... less some
> slight bits for inductive lag and ... well, speed of light 'n
> electricity 'n such, ... but also bit of mechanical inertia and such.
> But interesting too on the mechanical inertia, there's also critical
> damping (or nearly so) involved - and Eddy currents.  Well, there's
> that aluminum disk that spins - shows power consumption.  But how to
> have it stop, and not just keep sinning to only slowly slow down once a
> load stops?  Eddy currents.  Some permanent magnets are placed very
> close to that aluminum disk - that induces Eddy currents, which oppose
> the motion.  Works rather effectively as critical damping or fair
> approximation thereof - takes work to spin that disk between those
> magnets - that work is effectively a "motor" ... but quite effectively
> a W motor in this case - with speed proportional to the W it's
> measuring ... but when the W drops or is cut off, the disk still has
> those Eddy currents to slow it down - so it rather quickly slows or
> stops to reflect changes in power consumption - but still not quite
> instantaneous.  But, also being mechanical and all - and again,
> inertia, and Eddy currents, it also doesn't spin up or spin faster
> instantaneously with additional power consumption.  Those bits of not
> quite instantaneous pretty well cancel each other out, so the overall
> measured power consumption is quite accurate - and especially since
> it's mostly used for determining kilowatt hours on an approximately
> monthly basis, rather than instantaneous (kilo)watt power consumption.
> However, now with "Smart Meter"s and such, though still not
> instantaneous, often down to rather short intervals (e.g. 5 minute
> average, or less) of average power consumption are available.
>
> So ... Belkin Conserve Insight ... does it really measure and
> accurately display W, or is it really VA?  Don't know, but I'd
> guestimate it's probably W or a rather to quite good approximation
> thereof.  A known significantly out-of-phase load of sufficient size
> might be a way to easily determine that.  And, whether VA or W, in any
> case, how accurate?  Well, I do have test instruments that can rather
> accurately measure RMS A, and RMS V.  I could use that to get VA, but
> measuring W isn't as easy (I can't simply multiply those for W, as the
> measurements I'd have would be averaged rather than instantaneous).
>
> Oh, also, I don't think, in general, one will improve the accuracy of
> measuring power consumption of a small AC load, by starting with a
> larger load, adding a smaller load, then noting the difference in total
> power consumption.  Most notably, for most typical loads, the load
> won't be constant - notably varying by Voltage - which does fluctuate.
> E.g. that 7.5W incandescent night light one may have (before someone
> makes 'em illegal or whatever), it's actual power draw will likely vary
> a fair bit with voltage fluctuations, so adding a load such as that to
> try and determine the power draw of a smaller load, may actually give
> one less accurate information about the power draw of the smaller load.
> A more accurate approach would be to apply multiples of that smaller
> load (plug in many such identical devices under identical use status
> and conditions) ... but alas, that may not be feasible or convenient.
>
> Wee bit more on phase.  For larger commercial/industrial customers,
> power companies will often charge them an out-of-phase penalty or
> surcharge.  Why?  Because out-of-phase costs the power companies money.
> The power company's meter measures kilowatt-hours (kW-h) - or
> multiples thereof.  That's actual power consumed by customer.  However,
> out-of-phase energy is not free for the power companies to provide.
> Let's take a theoretical example.  Large commercial industrial customer
> hooks up a large purely inductive theoretical load to the power
> company meter.  Meter reads, and bills for 0 (zero) - no power consumed.
> But there is current and VA.  And the lines the power company uses to
> deliver the current - they're relatively long, for the most part, and
> they're not some theoretical zero resistance lines (though some power
> companies are doing some very limited trial stuff with power delivery
> over superconductors - at least from what I seem to recall having read
> some while back).  So ... current, resistance, volts ... power.  Power
> company in such case is paying (producing power) to heat up their
> lines, yet billing customer for zero power consumption.  Hence penalty
> for out-of-phase - to incentivize customer to be using in-phase
> current, and thus not only billable power, but more efficient for the
> power company, as that reduces waste current and waste heat on
> transmission lines, and optimally at fully in-phase, that waste is
> minimized.
>
>> Date: Wed, 18 Mar 2015 10:11:26 -0700
>> From: Ross Bernheim <rossbernheim at gmail.com>
>> To: Conspire List <conspire at linuxmafia.com>
>> Cc: Nick Moffitt <nick at zork.net>
>> Subject: Re: [conspire] check out AC power (Watt)meter (What's Your
>>     Watt?/...) from    library
>> Message-ID: <584DF350-97C9-43EF-8C9A-68CC76B69E5A at gmail.com>
>>
>> One thing to remember about the Kill A Watt and similar consumer
>> power/watt meters
>> is that they are meant for lights and appliances and while they will
>> easily measure
>> a refrigerator or 100W light bulb, they are not accurate at low power
>> levels.
>>
>> The accuracy at low levels, typically below seven watts is not
>> terribly useful. As
>> we move to LED light bulbs that only draw a few watts and very low power
>> computers with solid state drives these meters may give inaccurate
>> results.
>>
>> One suggestion is to use a multi-outlet power strip and pair the
>> device you want
>> to check with another device such as a light bulb that is a constant
>> load above
>> ten watts so that you can accurately measure it then add the load you want
>> to
>> test and measure the increased load accurately.
>>
>> All test equipment has limitations and you need to know what they are so
>> you can assure the accuracy of your measurements.
>>
>> Ross
>>
>>> On Mar 18, 2015, at 3:44 AM, Nick Moffitt <nick at zork.net> wrote:
>>>
>>> Michael Paoli:
>>>> One can check out AC power Wattmeter (What's Your Watt?) from
>>>> library.
>>>
>>> These devices are really useful for installations where you have a lot
>>> of devices running.  You can accurately measure median loads, sample and
>>> get a rolling average, and take note of peak load (typically on startup
>>> for devices that have spinning rust).
>>>
>>> It's also amazing to look at two identical pieces of hardware and notice
>>> that they have dramatically different power loads. Often you can trace
>>> that to software load differences, and make adjustments as necessary.
>>>
>>> But of course the best reason is for energy savings and capacity
>>> planning.  You can work out that your mains bus can handle N servers at
>>> normal load, and N/4 at peak load, or what have you.  Then you know to
>>> only start up a quarter of your systems at any one time, and stagger
>>> boots (or just increase capacity to cover full synchronised peak load).
>
>
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-- 
--
Best Regards.
This is unedited.
This message came out of me
via a suboptimal keyboard.