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Home Power Monitoring Hack
dvogt writes "You think your power bill is bad? I built a power monitoring system to monitor every circuit in my house with three second resolution for over a year. And while I had to rewire all my electrical to do it, I can now reconcile my electricity bill down to the penny... Of course when my wife figured out most of the bill was because of my computer gear I had to build her a dome, so reader beware!" From the article: "About a year ago I developed a web based power monitoring application for data centers. The application was designed to monitor thousands of individual branch circuits using current transducers at the breaker panels. Among other things, the data logging requirements were to provide one year of min/max/mean measurement data with one minute resolution per circuit. Since I had all the hardware for testing, I figured what better way to test things than to install it in my own home."
If you can read the wheel then you can test its accuracy. Just turn a known load on and off and measure the change.
You can do that with just a stop watch and some math.
The article on house wiring. http://www.kondra.com.nyud.net:8090/circuit/circui t.html
Another popular article from the site on building a ceiling dome. http://www.kondra.com.nyud.net:8090/dome/dome.html
I did my undergrad thesis in home power metering, and used 2 different models of Brand power meters. They sample current and voltage at 4kHz to accurately measure true power factor (see their description). Single circuit "plug-through" meters are $150-350; the higher-end ones have computer connectivity and datalogging.
Another option for those interested in exploring home power use--and not ignoring power factor--is the Watt's Up? meter (also plug-through, $100-150, with computer connectivity at the high-end). I believe both this and the Brand meter will show you the instantaneous power factor of the load you're metering if you cycle through the display.
While power factor might be irrelevant for simple inductive loads, I agree with the parent that this is an oversight (if it's true that the hardware in question doesn't do more than measure volts * amps), particularly if you want to check usage against your power bill. Power factor is more of issue for commercial electricity customers, who are billed for non-unity power factor and often install huge banks of capacitors to correct it (though i forget exactly how this helps).
I am not expert on the details of various building codes, but I am familiar with the intent of electrical codes that try to prevent high voltage/amperage wiring from being in the same enclosure as low stuff. For example, codes encourage 120 V wiring to run through conduits but prohibit running low power lines (such as phone) to run through the same conduits. Why? Because some stupid accident might cause the wires to become cross connected and blow out devices or start a fire.
Mounting an uncovered PCB (printed circuit board) that communicates with a computer within a 120 V distribution panel is a very big no-no. What if geek hubby is out of town and wifey experiences a power problem and calls in a yellow page electrician to fix the problem? In the worse case the "electrician" accidently drops a tool that winds up connecting 120 V to the computer circuits and starts a fire in the server room.
Building codes are designed as protection from stupidity - not only the stupidity of the the original builders but from the stupidity of those called in to fix problems.
To anybody who wants to do anything similar - it makes sense to put the current sensors in the distribution panel, but please rout them out to a seperate box that sends their info to a computer.
Power factor is more of issue for commercial electricity customers, who are billed for non-unity power factor and often install huge banks of capacitors to correct it
Here is a brief rundown and how it helps.
In DC Volts times Amps = Watts.
In AC Volta times Amps times Power factor = Watts.
Volts times Amps minus Watts = VAR's (Volt Amps Reactive) Power Factor is always between 0 and 1.0 and is either inductive or capacitive. 1.0 = no reactive current.
How does reactive power affect the power company?
Take for example an air conditioner. It's electric motor has windings that are inductive. The current is not directly in phase with the applied voltage. The current lags. The AC may draw 15 Amps, but on a 120 Volt circuit only consume 1200 Watts. 15 * 120 = 1800 Volt-Amps. 1200 Watts is the power used. 120 Volts * 15 Amps * 0.66667 PF = 1200 Watts. This leaves a component of 600 VAR's or 5 Amps of reactive current and a Power Factor of 0.66667 inductive. Drawing 5 Amps seems like no big deal to an end consumer. However for the power company, it means the transformer has an extra 5 amps as well as it's circuit breaker and wires. All wire has resiance. A current flowing in a wire will turn some of that voltage caused by the current to produce real Watts (heat) in the wire, transformer and circuit breakers. Your 5 Amps of 0 Watts costs the power company money to heat their lines and reduce their capicity.
Now the neat way to fix it. Capacitors don't heat (except for some small losses) Add some capacitors so the capacitive reactance = the inductive reactance on the line. In the above example 5 Amps is needed.
When done, the 5 Amps of capacitive reactance is out of phase with the inductive reactance by 180 degrees (90 degrees to the resitive load) and thus the 600 VAR's (5 Amps Inductive) from the AC is balanced with 600 VAR's (5 Amps Capacitive) from the capacitors. The nice thing is now the AC gets the current for the VAR's from the capacitors, not the power company. Now the AC uses only 10 Amps from the power company, not the 15 it used to. (The AC still draws 15 Amps, but the combined load of the AC and Capacitors is now 10 Amps and still 1200 Watts.) This is why the power company would like you to adjust your VAR's. If the power company tried to adjust it, (sometimes they do) by adding capacitors, then they may be unbalanced the other way (capacitive) when your AC shuts down but the capacitors don't.)
The truth shall set you free!
Speaking as a customer service rep at a electrical utility, I think it would be one of my more NORMAL calls. :)m
"NIST and Your Electricity Bill")
No one needs reminding about those meters with little turning wheels that the power company uses for determining your monthly electricity bills. It may be of some solace to know there is a third party out there who is helping to make sure those watt-hour meters, as they are known, accurately record the amount of power you are using. That way you don't pay for more electricity than you actually use, and, in all fairness, the power company doesn't end up giving away its products for free.
Typically speaking though, meters do not 'speed up.' With age, they slow down which does not work in the utility's favor, which is why they usually have a periodic meter replacement program, ie every 5 years.
I haven't ever sent out a service order on a meter that's ever tested too fast.
Many utility companies (like the one I work for) have automated metering, with meters transmitting at 917mhz back to a poletop box every 5 minutes for a second or so. (Like a cordless phone). What that means is that a lot of companies now have on their website a means for you to view your usage every day, so you can go online the next day after running your hot tub for a few hours and see how much extra you used in number and graph form.
But back to accuracy, typically, on a service order when a field rep goes out to test a meter, they will find the cause of the increased usage... like a new hot tub that was just installed
Meters all have to be tested to NIST standards... (from here: http://www.nist.gov/public_affairs/nhouse/watt.ht
This two-way assurance rests upon a short chain of calibrations anchored at the National Institute of Standards and Technology (NIST) where the ultimate power meter lives. The accuracy of every watt-hour meter in the country ultimately is traceable to the Electricity Division of NIST's Electronics and Electrical Engineering Laboratory. Most watt-hour meters are electromechanical devices in which a tiny portion of the electrical power going through it is converted into the mechanical clock-like motions that move the meter's dials. Just as clocks can be fast or slow, so too can watt-hour meters be off. That is why state public utility commissions (PUCs) own and maintain standard watt-hour meters with which they can certify the accuracy of mass-manufactured meters. Meters pass when they produce the same power reading as the standard meter when the same amount of current passes through them.
NIST provides the ultimate basis for these measurements because the standard meters of the PUCs go through periodic calibrations at NIST in which the amount of electricity going through a meter can be more accurately and confidently measured than anywhere else in the country. Once the standard watt-hour meters pass muster at NIST, they can serve as genuine gatekeepers for the much larger population of residential and business watt-hour meters.