# Power Calculation and Current Direction

Hey Guys. I’m relatively new to this and have an EmonPi that I am using for some PV production monitoring. I have a question about how power is calculated though because I never see any negative power values.

My emonpi CT is connected to a circuit that has a few small loads on it as well as 500W of 120V PV inverters (United States). I am using a 120V wall transformer for reference voltage. I would expect to see power production during the day as well as consumption at night but this hasn’t been what I have been seeing. I have been seeing always a positive power number. I suspect that this is due to the way that power is calculated. Is there a way to change this or do I need to just monitor my PV production differently than my loads?

Sorry if this is a repetitive question. I have been searching and haven’t found the solution.

The way power is calculated will and does produce negative values when the direction of power flow reverses.
So my first guess is, you are not measuring the current where the power does indeed reverse. So where are you measuring the current? Is it really where you think it is? It must be between where your service entrance wires enter your property, and the point where the PV infeed is connected.
My second guess is you might have connected the a.c. reference voltage after you powered up your emonPi. If you did, it’s decided it hasn’t got an a.c. voltage input, and it’s ignoring it. Then it estimates apparent power using the nominal voltage, and that will never be negative. The solution to that is power down your emonPi (a reset is not good enough) and power up with the a.c. input connected.

Well that was an easy solution! I did connect the A.C. reference voltage after i powered up the unit. Power cycling solved my problem! Thanks again!

It is actually in the FAQ, but I think you didn’t recognise the symptom:
Q: My emonTx / emonTH / emonPi won’t read the new sensor that I just plugged in. How do I fix it?