Today I had my system up an running. I have the emonTx Shield V2.
First I started in standard mode, measuring a steady 90 Watt on a particular branch.
Next I tried the 'real power' mode. I used the other arduino sketch. And I noticed 2 things:
First, the output was negative. This could be fixed by turning the 9VAC adapter the other way in the power socket. Pleas note that I'm from the netherlands. Power plugs can be inserted 'upside-down', due to the kind of power plugs we use here. When I reversed the power adapter in its socket, the power became a positive value.
The second thing is the value. Now I measured around 60 W whereas it was around 90 W with the old method. Quite a large difference. Not that I didnt use the power adapter from this webshop, but one that I still had lying around.
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Could this make the difference?
Or is there a way to calibrate the result? I was thinking of measuring the total power consumption during a full day and comparing the nr of kwhrs to the ones on the power meter in the closet.
Re: Real Power
First, the output was negative. This could be fixed by turning the 9VAC adapter the other way in the power socket.
Sounds like you had your CT and your VT out of phase. Flipping either will fix the problem, so if you'd prefer to run with your adapter the right way up, simply flip the CT on the wire it's measuring. If neither of those solutions is convenient, simply multiply the result by -1.
Or is there a way to calibrate the result?
Yes, you always need to calibrate:
http://openenergymonitor.org/emon/buildingblocks/calibration