<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div dir="ltr" id="yui_3_16_0_1_1429716478493_44842"><font id="yui_3_16_0_1_1429716478493_45668"><span id="yui_3_16_0_1_1429716478493_45035">Michael,</span></font></div><div id="yui_3_16_0_1_1429716478493_44858" dir="ltr"><br><span></span></div><div id="yui_3_16_0_1_1429716478493_44886" dir="ltr"><font id="yui_3_16_0_1_1429716478493_45231"><span id="yui_3_16_0_1_1429716478493_44887">Thank you for the very good explanation of the subtleties of AC power.</span></font></div><div id="yui_3_16_0_1_1429716478493_44928" dir="ltr"><br><span id="yui_3_16_0_1_1429716478493_44887"></span></div><div id="yui_3_16_0_1_1429716478493_44930" dir="ltr"><font id="yui_3_16_0_1_1429716478493_45144"><span id="yui_3_16_0_1_1429716478493_44887">But the manual does indeed have a section on specifications:</span></font></div><div id="yui_3_16_0_1_1429716478493_44932" dir="ltr"><br><span id="yui_3_16_0_1_1429716478493_44887"></span></div><div id="yui_3_16_0_1_1429716478493_44934" dir="ltr"><span id="yui_3_16_0_1_1429716478493_44887"> Specifications<br style="" class=""></span><span id="yui_3_16_0_1_1429716478493_44887"> 5 ft. cord connecting display and socket</span><br style="" class=""><span id="yui_3_16_0_1_1429716478493_44887"> Electrical Rating: 120V~/15A/60Hz/1800W</span><br style="" class=""><span id="yui_3_16_0_1_1429716478493_44887"> Maximum Watts Displayed: 1800</span><br style="" class=""><span id="yui_3_16_0_1_1429716478493_44887"> Maximum Dollars Displayed: 9999</span><span id="yui_3_16_0_1_1429716478493_44887"></span></div><div id="yui_3_16_0_1_1429716478493_45033"><br></div><div dir="ltr" id="yui_3_16_0_1_1429716478493_45037"><font id="yui_3_16_0_1_1429716478493_45177">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 w</font>hat they are hiding.<br></div><div id="yui_3_16_0_1_1429716478493_45364" dir="ltr"><br></div><div id="yui_3_16_0_1_1429716478493_45366" dir="ltr">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. <br></div><div id="yui_3_16_0_1_1429716478493_45551" dir="ltr"><br></div><div id="yui_3_16_0_1_1429716478493_45599" dir="ltr">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".<br></div><div id="yui_3_16_0_1_1429716478493_45363" dir="ltr"><br></div><div id="yui_3_16_0_1_1429716478493_45039"><br> </div><div id="yui_3_16_0_1_1429716478493_44937" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;"> <div id="yui_3_16_0_1_1429716478493_44936" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div id="yui_3_16_0_1_1429716478493_44935" dir="ltr"> <hr id="yui_3_16_0_1_1429716478493_45001" size="1"> <font id="yui_3_16_0_1_1429716478493_44941" face="Arial" size="2"> <b id="yui_3_16_0_1_1429716478493_45169"><span id="yui_3_16_0_1_1429716478493_45168" style="font-weight:bold;">From:</span></b> Michael Paoli <Michael.Paoli@cal.berkeley.edu><br> <b id="yui_3_16_0_1_1429716478493_45382"><span id="yui_3_16_0_1_1429716478493_45381" style="font-weight: bold;">To:</span></b> conspire@linuxmafia.com <br> <b id="yui_3_16_0_1_1429716478493_45379"><span id="yui_3_16_0_1_1429716478493_45378" style="font-weight: bold;">Sent:</span></b> Sunday, April 12, 2015 10:21 AM<br> <b id="yui_3_16_0_1_1429716478493_45376"><span id="yui_3_16_0_1_1429716478493_45375" style="font-weight: bold;">Subject:</span></b> [conspire] How accurate, etc.: AC Power (Watt)meter (What's Your Watt?/...)<br> </font> </div> <div id="yui_3_16_0_1_1429716478493_44939" class="y_msg_container"><br>Well ... it varies. ;-)<br><br>But yes, good points - not so much that "they" or "it" is necessarily<br>inaccurate or that inaccurate, but good to generally be aware of how<br>(in)accurate one's measurements are.<br><br>Most test instruments typically have specifications, that well spell out<br>their accuracy. And yes, the specifications generally are quite<br>accurate - and especially from reputable manufacturing companies. That<br>doesn't mean the specifications are always 100% correct, but they're<br>generally intended to be correct, and for the most part are.<br><br>That being said, some related bits ...<br><br>First, I was curious on the "What's Your Watt" - specifications? How<br>accurate *is* it?<br>A trace of sleuthing and I found ...<br><a href="http://www.sfenvironment.org/whatsyourwatt" target="_blank">http://www.sfenvironment.org/whatsyourwatt</a><br><a href="http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf" target="_blank">http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf</a><br>... apparently Belkin Conserve Insight<br>... as far as I can tell, seems they have, for USA(/Canada) power<br>compatible versions, 3 very slightly different versions of this device:<br>Part # F7C005 <a href="http://www.belkin.com/us/support-product?pid=01t80000002zwZFAAY" target="_blank">http://www.belkin.com/us/support-product?pid=01t80000002zwZFAAY</a><br>Part # F7C005q <a href="http://www.belkin.com/us/support-product?pid=01t80000002HrhcAAC" target="_blank">http://www.belkin.com/us/support-product?pid=01t80000002HrhcAAC</a><br>Part # F7C005fc <br><a href="http://www.belkin.com/us/support-product?pid=01t80000002If1gAAC" target="_blank">http://www.belkin.com/us/support-product?pid=01t80000002If1gAAC</a><br>But I also notice, all 3 have the exact same User Manual:<br>Conserve Insight F7C005 v1 - User Manual (US) (English)<br><a href="http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf" target="_blank">http://cache-www.belkin.com/support/dl/web_f7c005_10ms041_conserveinsight.pdf</a><br>which is exact same URL also found further above.<br>In all the checks, I didn't find specifications that covered the<br>accuracy of its power measurements! - somewhat surprising. But it does<br>give wee bit about it its inaccuracy at (very) low power levels:<br>"<br>Your Conserve Insight uses sensitive<br>electronics to measure the amount of power<br>flowing to your connected device. When<br>the connected device is consuming a very<br>small amount of electricity, it becomes<br>very difficult to distinguish between the<br>power being consumed and the electrical<br>noise found at all power outlets. To ensure<br>accurate readings, the Conserve Insight<br>stops showing values below 0.5 watts and<br>displays the '0-0.5 Watt' screen.<br>"<br><br>Anyway, notwithstanding the lack of more detailed specification (it does<br>gives specifications, but alas, doesn't state how accurate the power<br>measurements are), it is from a reputable instrument manufacturer, so<br>it's probably *fairly* accurate ... at least compared roughly to<br>comparable test instruments (e.g. electronic measurement instrument in<br>roughly the $15 to $50 USD price range ... and in probably about 2012 or<br>so). So, a good guestimate might be something in the range of roughly<br>+-2% to +-5%, or so. Since Belkin also quite implies it can't<br>accurately measure <~=0.5 Watts (W), one might guestimate accuracy of<br>something like the worse of:<br>+-2% to +-5%, or 0.5W, whichever is greater.<br><br>But more on how (in)accurate. Measuring Watts isn't as simple as<br>measuring Volt-Amperes(Amps) (VA). Watts - or true power consumption,<br>is the *instantaneous* product of Volts and Amps - and since that<br>continuously fluctuates for Alternating Current (AC), that's generally<br>given averaged over some reasonable interval (one or more full cycles,<br>e.g. for 60 Hertz ((Hz) Cycles Per seconds - CPS), one second would<br>cover 60 cycles). The difference between VA and Watts, is VA is mere<br>product of the Root Mean Square (RMS) values for each of V and A. Note<br>that VA and Watts can give *very* different numbers. W is the true<br>power consumption, whereas VA is not - though knowing or also knowing<br>VA can be useful in certain contexts, and sometimes/commonly, VA may be<br>a rough, to even good, approximation of W (and knowing the VA is<br>generally better than having no W information). Without explaining RMS<br>in detail, think of it as "equivalent". If one has a pure resistive<br>load, let's say of 1 Ohm, applies a Direct Current (DC) Voltage of 1 V,<br>then one has current of 1A, and power consumption of 1W. If instead of<br>DC, and AC voltage is applied, regardless of what waveshape applied, if<br>the AC RMS voltage is 1V, the RMS current will be 1A, and the power<br>will be 1W (at least averaged over any full cycle). Note also here,<br>talking theoretical resistor (same resistance regardless of<br>current/voltage). Anyway, that's a key reason why RMS values are so<br>useful - for their relative equivalence. Better electrical instruments<br>will give true RMS values, other less accurate ones will give an<br>estimate, typically taking some other measurement and presuming the<br>waveshape is sinusoidal (most commonly it's fairly close to sinusoidal<br>- particularly for common power situations, but that's not always the<br>case), and will then give a reading based upon that presumption.<br><br>Anyway, W vs. VA ... phase matters. The more out-of-phase they are,<br>the more they'll differ. Bit of crude analogy - think of pushing<br>someone on a swing - regularly push as they start the downward portion<br>- that's in phase - adding energy to the system - to build their<br>swinging - or at least offset losses from various friction and such.<br>If instead, one pushes as they're approaching towards the top of their<br>swinging motion towards you, that's out-of-phase, and instead of adding<br>energy to the system, is taking energy out, and reducing their<br>swinging. One might be doing the same amount of pushing in either<br>case, but not only how efficiently energy is flowing - but even which<br>direction - depends on the phase relationship. Such is the case with<br>AC. Measuring one's pushing, while ignoring phase, would be akin to<br>measuring VA (or at least one of its components), rather than properly<br>measuring and determining W.<br><br>Ye olde power company electric meters. I forget the precise details of<br>how they do it (I read the description many decades ago), but they very<br>much measure W, not VA. They very effectively and quite efficiently,<br>do a continuous "instantaneous" electromechanical multiplication of the<br>V and A - and thus measure true W. Well, ... *almost* instantaneous.<br>The electrical field parts of it they use to do that are ... less some<br>slight bits for inductive lag and ... well, speed of light 'n<br>electricity 'n such, ... but also bit of mechanical inertia and such.<br>But interesting too on the mechanical inertia, there's also critical<br>damping (or nearly so) involved - and Eddy currents. Well, there's<br>that aluminum disk that spins - shows power consumption. But how to<br>have it stop, and not just keep sinning to only slowly slow down once a<br>load stops? Eddy currents. Some permanent magnets are placed very<br>close to that aluminum disk - that induces Eddy currents, which oppose<br>the motion. Works rather effectively as critical damping or fair<br>approximation thereof - takes work to spin that disk between those<br>magnets - that work is effectively a "motor" ... but quite effectively<br>a W motor in this case - with speed proportional to the W it's<br>measuring ... but when the W drops or is cut off, the disk still has<br>those Eddy currents to slow it down - so it rather quickly slows or<br>stops to reflect changes in power consumption - but still not quite<br>instantaneous. But, also being mechanical and all - and again,<br>inertia, and Eddy currents, it also doesn't spin up or spin faster<br>instantaneously with additional power consumption. Those bits of not<br>quite instantaneous pretty well cancel each other out, so the overall<br>measured power consumption is quite accurate - and especially since<br>it's mostly used for determining kilowatt hours on an approximately<br>monthly basis, rather than instantaneous (kilo)watt power consumption.<br>However, now with "Smart Meter"s and such, though still not<br>instantaneous, often down to rather short intervals (e.g. 5 minute<br>average, or less) of average power consumption are available.<br><br>So ... Belkin Conserve Insight ... does it really measure and<br>accurately display W, or is it really VA? Don't know, but I'd<br>guestimate it's probably W or a rather to quite good approximation<br>thereof. A known significantly out-of-phase load of sufficient size<br>might be a way to easily determine that. And, whether VA or W, in any<br>case, how accurate? Well, I do have test instruments that can rather<br>accurately measure RMS A, and RMS V. I could use that to get VA, but<br>measuring W isn't as easy (I can't simply multiply those for W, as the<br>measurements I'd have would be averaged rather than instantaneous).<br><br>Oh, also, I don't think, in general, one will improve the accuracy of<br>measuring power consumption of a small AC load, by starting with a<br>larger load, adding a smaller load, then noting the difference in total<br>power consumption. Most notably, for most typical loads, the load<br>won't be constant - notably varying by Voltage - which does fluctuate.<br>E.g. that 7.5W incandescent night light one may have (before someone<br>makes 'em illegal or whatever), it's actual power draw will likely vary<br>a fair bit with voltage fluctuations, so adding a load such as that to<br>try and determine the power draw of a smaller load, may actually give<br>one less accurate information about the power draw of the smaller load.<br>A more accurate approach would be to apply multiples of that smaller<br>load (plug in many such identical devices under identical use status<br>and conditions) ... but alas, that may not be feasible or convenient.<br><br>Wee bit more on phase. For larger commercial/industrial customers,<br>power companies will often charge them an out-of-phase penalty or<br>surcharge. Why? Because out-of-phase costs the power companies money.<br>The power company's meter measures kilowatt-hours (kW-h) - or<br>multiples thereof. That's actual power consumed by customer. However,<br>out-of-phase energy is not free for the power companies to provide.<br>Let's take a theoretical example. Large commercial industrial customer<br>hooks up a large purely inductive theoretical load to the power<br>company meter. Meter reads, and bills for 0 (zero) - no power consumed.<br>But there is current and VA. And the lines the power company uses to<br>deliver the current - they're relatively long, for the most part, and<br>they're not some theoretical zero resistance lines (though some power<br>companies are doing some very limited trial stuff with power delivery<br>over superconductors - at least from what I seem to recall having read<br>some while back). So ... current, resistance, volts ... power. Power<br>company in such case is paying (producing power) to heat up their<br>lines, yet billing customer for zero power consumption. Hence penalty<br>for out-of-phase - to incentivize customer to be using in-phase<br>current, and thus not only billable power, but more efficient for the<br>power company, as that reduces waste current and waste heat on<br>transmission lines, and optimally at fully in-phase, that waste is<br>minimized.<br><br>> Date: Wed, 18 Mar 2015 10:11:26 -0700<br>> From: Ross Bernheim <<a ymailto="mailto:rossbernheim@gmail.com" href="mailto:rossbernheim@gmail.com">rossbernheim@gmail.com</a>><br>> To: Conspire List <<a ymailto="mailto:conspire@linuxmafia.com" href="mailto:conspire@linuxmafia.com">conspire@linuxmafia.com</a>><br>> Cc: Nick Moffitt <<a ymailto="mailto:nick@zork.net" href="mailto:nick@zork.net">nick@zork.net</a>><br>> Subject: Re: [conspire] check out AC power (Watt)meter (What's Your<br>> Watt?/...) from library<br>> Message-ID: <<a ymailto="mailto:584DF350-97C9-43EF-8C9A-68CC76B69E5A@gmail.com" href="mailto:584DF350-97C9-43EF-8C9A-68CC76B69E5A@gmail.com">584DF350-97C9-43EF-8C9A-68CC76B69E5A@gmail.com</a>><br>><br>> One thing to remember about the Kill A Watt and similar consumer <br>> power/watt meters<br>> is that they are meant for lights and appliances and while they will <br>> easily measure<br>> a refrigerator or 100W light bulb, they are not accurate at low power levels.<br>><br>> The accuracy at low levels, typically below seven watts is not <br>> terribly useful. As<br>> we move to LED light bulbs that only draw a few watts and very low power<br>> computers with solid state drives these meters may give inaccurate results.<br>><br>> One suggestion is to use a multi-outlet power strip and pair the <br>> device you want<br>> to check with another device such as a light bulb that is a constant <br>> load above<br>> ten watts so that you can accurately measure it then add the load you want to<br>> test and measure the increased load accurately.<br>><br>> All test equipment has limitations and you need to know what they are so<br>> you can assure the accuracy of your measurements.<br>><br>> Ross<br>><br>>> On Mar 18, 2015, at 3:44 AM, Nick Moffitt <<a ymailto="mailto:nick@zork.net" href="mailto:nick@zork.net">nick@zork.net</a>> wrote:<br>>><br>>> Michael Paoli:<br>>>> One can check out AC power Wattmeter (What's Your Watt?) from<br>>>> library.<br>>><br>>> These devices are really useful for installations where you have a lot<br>>> of devices running. You can accurately measure median loads, sample and<br>>> get a rolling average, and take note of peak load (typically on startup<br>>> for devices that have spinning rust).<br>>><br>>> It's also amazing to look at two identical pieces of hardware and notice<br>>> that they have dramatically different power loads. Often you can trace<br>>> that to software load differences, and make adjustments as necessary.<br>>><br>>> But of course the best reason is for energy savings and capacity<br>>> planning. You can work out that your mains bus can handle N servers at<br>>> normal load, and N/4 at peak load, or what have you. Then you know to<br>>> only start up a quarter of your systems at any one time, and stagger<br>>> boots (or just increase capacity to cover full synchronised peak load).<br><br><br>_______________________________________________<br>conspire mailing list<br><a ymailto="mailto:conspire@linuxmafia.com" href="mailto:conspire@linuxmafia.com">conspire@linuxmafia.com</a><br><a href="http://linuxmafia.com/mailman/listinfo/conspire" target="_blank">http://linuxmafia.com/mailman/listinfo/conspire</a><br><br><br></div> </div> </div> </div></body></html>