Appliance kiloWatt-hours measuring

[zampaq]

Im not much of an electronics hobbyist. I did a lot of kit building when i was young and worked at Rod Irving Electronics in Melbourne. I dont even think RIE's still around. I want to know if there is a safe project that can be built to show how many KWH are being used by an appliance at any moment.eg, I have my computer set to turn off hard drive and monitor after 5 mins of no use. Id like to know how much power the computer is using in both awake and sleep modes. There are lots of things I have that Id like to know how much power they are exactly using. I kind of picture something with male and female mains plugs connected like an extention cord with a box with controls workings and interface, set in the middle.
Does anyone know anything about the circuitry involved. Cheers

 

[daddles]

I'd imagine few people build such things since a tool like the Kill-A-Watt is available. I got mine in the US for $20 and it gets heavily used.

 

[(*steve*)]

There are so many available so cheaply that it's hardly worth the effort.

I got one free for purchasing some relatively small amount of stuff from Rockby not long ago.

Anything that uses a current clamp to do the measuring will suffer from not being able to measure power factor (and almost certainly thus be incapable of taking it into account). This is a major issue with measuring the power consumed by your PC since it may have a power factor of around 0.7.

edit: if it doesn't have a current clamp it will require direct connection to the mains which clearly means you need to take a lot more care in construction.

 

[donkey]

also here in qld we have a promotion where the government gives you an energy monitor and a remote for switches for 50bucks. check to see if your government has something similiar going. awesome savings to be made

 

[rootboy]

I've got a number of these:

http://www.schneider.nl/docs/Power Meter PM800 technical data sheet.pdf

and one of these:

http://www.powernet-mcl.com/pml/8500.php

at my desk, but I'm not allowed to hand out free samples.

As for the circuitry... Hmmm, good question. I had one of the PM8xx's apart the other day robbing a pin out of it to fix another PM850. I'll try to remember to take the "bad" one apart and see if I can figure out what makes it tick. Right now I'm wiring up some switchgear, and I have to get back to it. Oh, the humanity....