My electric utility released an iPhone app called "DTE Insight" which (among other things) measures the amount of power flowing through an appliance's power cord by asking you to hold the cord against a specific part of the iPhone at a specific angle. I tested this app against the measurements from my Kill-a-Watt, and the app seems remarkably accurate to the point where I don't really understand how it works.
I understand that current flowing through a conductor creates a magnetic field around the conductor, and I imagine that this app is using the phone's magnetic field sensor to try to measure the magnetic field caused by the current flowing through the cable, then multiplying the calculated current by an assumed voltage of 115 or 120 or whatever in order to estimate power.
What confuses me is that household appliance cables contain two current-carrying conductors. Wouldn't the opposing magnetic fields from the two conductors interfere with and almost completely cancel out their respective fields out in the area around the cable? The app does ask whether the cord being tested is a two- or three-pronged cord, and whether the cord is round or flat. Can anybody explain to me how this might work?
I did some tests using the app and my Kill-a-Watt to measure power draw of the same loads. I did five trials of each of the four loads using the app. The loads were:
- Assorted computing equipment sharing a power strip
- An electric fan on low
- An electric fan on medium
- An electric fan on high
The results are as follows:
Kill-a-Watt: 632 W, 757 VA
App: 678 W, 667 W, 623 W, 662 W, 644 W
Fan on Low:
Kill-a-Watt: 56 W, 57 VA
App: 75 W, 75 W, 75 W, 75 W, 77 W
Fan on Medium:
Kill-a-Watt: 75 W, 75 VA
App: 103 W, 101 W, 98 W, 100 W, 97 W
Fan on High:
Kill-a-Watt: 97 W, 102 VA
App: 132 W, 129 W, 131 W, 128 W, 131 W
If you want to try this yourself, you have to select "Tools" and then "Take a Power Scan" from the main menu of the app.