Monitor (and trend) power usage of appliances

[jderou]

I started looking into building a solar cell, but after some calculation it looks like it would take 2 years of sun to payback an investment of $100. Sooo, I think it would be better spent knowing what is on in the house, and what it is costing.

I want to be able to monitor power usage from several appliances in the house, and I want to trend it on a pc.
Doing this would make it easy to see where most of our money is going, and calculate a payback on replacing the appliance (or whatever).
Anyone have a sample circuit I could make to tie this in to a PC?
Are there analog input cards I could find cheaply?

Thanks for any ideas!

[doorknob]

How accurate do you want it to be?

It's probably a lot easier to simply monitor AC current rather than trying to measure actual power (taking into account both voltage and current as well as the phase relationship between them).

If you just want to measure current, then you can get a PC-interface multimeter (either USB or RS-232) with an AC current probe (that you can clamp around one conductor of an AC power line) and sample it with simple data collection software. You wouldn't have to do any analog-to-digital conversion, rather that would be handled by the multimeter.

I use a Kill-A-Watt meter for doing such measurements (actually I have two different models that I use), but that meter does not have an output for use with an automated PC data collection system. It is possible that a place such as smarthome.com has power monitors with PC interfaces, but I have no experience with any such units.

[jderou]

Current is fine, cause I can write the software to convert to power. How cheap do you think you could find a multimeter with a pc interface? Probably too high to justify buying four or five. Maybe I could just trend for a week and move on to something else. That would probably tell me enough.
I did see the kill-a-watt units, they even have wireless units with a base. Can't believe they don't have a pc interface on that one.

[jderou]

Wonder how accurate one of these would be:
Digital Multimeter Electronic Tester AC/DC CLAMP Meter | eBay

I may be able to hack into it and steal the info going to the LCD, somehow tie it into the pc.

[doorknob]

Occasionally Harbor Freight has a clamp-on meter for sale for as little as $9.00.

This one is currently (excuse the pun) for sale at $13.00, so with a 20% off coupon that would bring it down to $10.40 (if you have a local HF store):

Digital Clamp Meter

While I've never actually tried to demultiplex the inputs to an LCD in order to extract the display data, I have considered the possibility of doing so, and expect that it might possibly be do-able. I'd expect that the guts of such a meter would be tightly integrated, so there may not be any other place where you could tap off a more straightforward reading.

Thinking outside of the box here, it should be possible to do a non-invasive method of data collection by doing OCR on the numbers shown on the display (using a webcam, for example). But that would raise instead of lower the complexity, and you would be unlikely to be able to group several multimeter displays within the field of view of a single camera.

If you do want to try actually doing an A/D conversion from a clamp-on current sensor, take a look at this sensor from SparkFun:

Non-Invasive Current Sensor - 30A - SparkFun Electronics

It can be used in an Arduino energy monitor project, so maybe that's what you would want to pursue, but it might not be inexpensive enough for your purposes (so maybe that puts you back to running multiple current sensors into a single PC A/D card, or perhaps instead setting up one inexpensive data logger per current sensor, and then consolidating the data on one PC for later analysis):

Mains AC: non-invasive 3.0 | OpenEnergyMonitor

[bam4800]

Call your power company to find out if/when they plan to implement continuous monitoring of your electricity usage, and ask how you can access the data from your meter.

[fizzissist]

Call your power company to find out if/when they plan to implement continuous monitoring of your electricity usage, and ask how you can access the data from your meter.

The very LAST thing in the world is wanting the power company to monitor your usage. As soon as they can profile, it, there's ONE MORE EXCUSE to raise your rates.

Whether it's the "cost" of the equipment to monitor, the "cost" of acquiring and managing the data, the "cost" of billing, or the "cost" of administration for splitting the billing into different tiers.... It's just one more excuse to INCREASE your bill.

If you really need to know, get a hobbs meter, or build a stupid simple chart recorder. Don't let the power company in your house, in your life.