Watts from a Smart Plug?

Python
1

New to the digi products… Using an X2 and a Smart Plug.

I see this code in the smart_plug.py sample on the wiki:
http://www.digi.com/wiki/developer/index.php/Smart_Plug_Interactive_Demo


parsed_sample = parseIS(raw_sample)    
lux = (float(parsed_sample["AI1"]) / 1023.0) * 1200.0
mV = (float(parsed_sample["AI3"]) /1023.0) * 1200.0
current = (mV*(156.0/47.0) - 520.0) /180.0 * .7071 
return (lux, current)

Is there any explanation about these numbers?

I see 1200 is used to convert the AD reading of millivolts on AI1 and AI13 because the analog inputs go from 0VDC to 1.2VDC, right? But what about:

156/47?
520/180?
.7071? (power factor, maybe?)

I’m interested in deriving power in watts but at this point I assume I’m out of luck.

I’ve got a 60 Watt light bulb on the Smart Plug.
I can turn it on:
mV = 198.240469208 - Current = 0.542069312202

I can turn it off:
mV = 160.703812317 - Current = 0.0526392254737

With a separate voltmeter I can see the AC voltage on the power strip is around 122V. That works out to:
On = 122 * 0.542069312202 or 66.13… Watts
Off = 122 * 0.0526392254737 or 6.42… Watts

Looks like I’m off by 6 watts or so. IOW, if I shave off 6 watts from my on/off readings, I would have what I would expect; 60 watts on, and 0 watts off.

So the question is: Does anyone use this device for power measurements and if so, how?

I’ve been using some hacked Kill-a-watt meters (see Tweet-a-watt) and an Asus WL520GU wifi router (flashed w/ OpenWRT and python) with some reasonable success so I thought I’d try a professional product and see how it works. Now wondering why I’d pay $90 for Smart Plugs.

Any tips greatly appreciated,
Chris.

2

Well, if I can’t find any info on the digi site and no one here knows where to find the documentation, this is very disappointing.

Am I the only one who wants to track watts at the plug load level through an X2?

I was under the impression that I could use this device to track energy usage at the plug level. I tested different plug voltages in my home and it ranges from 120 to 123. I can’t just use a fixed number in my calculations so I’ll need to find a way to detect voltage for each smart plug outlet. Tedious at best.

Oh, well…

3

Are there any zigbee based devices that track plug load watts? Other than the Twee-a-watt? :wink:

TIA,
Chris.