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."

Comments

Another way to do it: read the meter

[seanadams.com]

If you want to monitor the whole house (as opposed to individual circuits) you can do it for less than $20 in parts plus a Linux machine. I made a system to do this a couple years ago - unfortunately I never hooked it up again after I moved, but it worked just fine.

If you're lucky enough to have the kind of electric meter with a blinking LED on it, you could do this much more simply. Also if I had to do this again I would ditch the op-amp circuit and feed the signal from the photo-resistor straight into the sound card and then do the filtering in software (if the photo-resistor is exposed to sunlight it can be a little tricky to tune using this circuit - software could be smarter).

Re:Another way to do it: read the meter

[d2_m_viant]

Yes, but that won't do much if the power company is cheating you. The article's way, you'd be more able to catch the power company charging you for something you didn't use.

On a side note: Imagine trying to convince the customer service rep on the phone that you rewired your house with a homemade power monitoring system and your monthly audits of your electrical usage uncovered the error...me thinks you'd have better luck convincing a Slashdot reader to install the WeatherBug...

electrical bill now?

[AmigaAvenger]

great, you do all this to minimize your bill, then post on slashdot?!?! server is already slow, I imagine the meter is spinning like a top right now...

Have to say

[zegebbers]

Since I had all the hardware for testing, I figured what better way to test things than to install it in my own home.

That's an awesome attitude that we don't get enough of on slashdot these days :(

Re:Does your home still meet safety codes?

[CyricZ]

Just because it is working now does not mean that it will be in several weeks, months or years. Such codes are in place to prevent home hackery like this, especially in the case of electrical wiring. The potential for his modifications to fail are astronomical. And if he did such modifications without proper inspections, then the situation could get very hairy were things to go wrong.

Perhaps his insurance company would not pay him if it was found that his uninspected electrical modifications were the cause of his house burning down, for instance.

coral cache links

[AMystery]

Since I had to go look for caches, i thought I'd share.

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

Yeah

[cynical kane]

You don't mess with a woman with a Power Management System...

(ducks)

Re:Does your home still meet safety codes?

[ErikZ]

Such codes are in place to prevent home hackery like this, especially in the case of electrical wiring. The potential for his modifications to fail are astronomical.

I agree! Think of the dangers of brewing your own beer, working on your car, or even programming! Almost all viruses were programmed without any kind of government oversight.

There should be codes that are enforced by the governemnt that touch upon everything you can possibly do. To protect you, and others, from yourself.

Seriously, the codes are there to protect people from substandard wiring. But to insist that you have no right to modify the wiring in your own house is too "Big Brother" for me.

1 minute resolution is not enough

[zippthorne]

In fact he must sample at greater than 120hz* to get meaningful results. He has neglected the possibility that voltage and current can and will be out of phase for each of the loads in his house. Without determining the phase difference, there is no way to accurately deterimne the average power over any interval.

There are quite a few meters that measure RMS voltage and RMS current, (though most of the cheap ones actually measure peak values and multiply by .707), there are fewer still that accurately resolve power factor

This is a common mistake to make for first year EE students and "over-unity" power converter proponants.

As I understand it, the Kill-A-Watt, http://www.professionalequipment.com/xq/ASP/Produc tID.3375/id.5/subID.57/qx/default.htm makes a pretty good approximation. In fact, it even does the integration for you. You could pepper every outlet with these things or just move them around as needed.

*I know you need 2f according to nyquist to resolve the frequency, but I'm not sure what you need to gather the phase information**

** There are other ways to obtain the phase information involving bridge circuits and such, It does not appear that the boards in question provide that information.

Re:1 minute resolution is not enough

[dan42]

I believe the 1 minute resolution was for the reported data and not the actual analog sampling rate. The 1 minute data may (should) be the RMS of the all the samples collected during that minute.
His device seems to be a prototype version of the Veris H663. The released version of the device apparently samples at 1280Hz and reports data every second.
The "phase information" you mention is good to have, but only if the power is at just one frequency. The fact is that computers (as with most devices running of a switching power supply) draw current as a short pulse during the peaks of the input voltage waveform (with no phase angle between voltage and current at the fundamental frequency). Often the 60Hz component (or whatever the fundamental frequency) makes up only half the RMS current. And since these harmonics are only dominant in the current and not the voltage waveform, the real power consumed by a computer will typically be ~50% x RMScurrent x RMSvoltage (even though the phase angle is zero).

Re:1 minute resolution is not enough

[Technician]

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.)

Re:From the article...

[gkuz]

Can you imagine being quizzed by yourhusband about how many loads of laundry you did a certain day and then having him insist you come with him to see on the computers about how you were WRONG and you were OFF BY ONE?

I've been married for 28 years, largely by studiously avoiding that sort of behavior. I'm frankly surprised the guy is still alive, let alone still married.

mandatory wtf....

[Anonymous Coward]

Hardware: Zero dollars
Software: Zero dollars
Linux box: Zero dollars

Explaining to your wife that after hours of development you've built a device that proves the most power hungry appliance in your house is the damn power monitoring system itself: Priceless