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Negative power Value after adding AC Voltage Sensor

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Hi New user here, I have been setting up my just purchased, emonpi bundle.

Last night I installed the CT1 on my Live main feed and since then I have been getting data from it. This evening I plugged in my AC Sensor and the main feed data is now showing as negative.

I know that is the CT is the wrong way round it can give negative readings but I have not changed that and something has caused it to flip. Unfortunately its very difficult for me to access the sensor now as I have to remove a load of covers and ducts to access the cable.


  1. How does plugging in the AC Sensor cause the power reading to flip negative?
  2. Is there a way for me to flip the reading to positive for example via a setting (I have looked for a setting but can’t find one).


Welcome, Tim, to the OEM forum.

Very simply, before you plugged in the a.c. sensor, the emonPi had no means of knowing the voltage, so (a) it assumed 230 V (yes, I know the UK centre voltage is still 240 V) and then it calculated the “apparent power”, which is always positive. Then, when it had got the voltage, it could determine the direction of the current in relation to the voltage, and so the direction of power flow, and more importantly it could calculate “real power”, which is what you pay for. There’s a full explanation on the first page of the ‘Learn’ section about electricity monitoring: Learn→Electricity Monitoring→AC Power Theory→Introduction→An Introduction to AC Power

There is a way, but there’s not a setting for it. In emonCMS, on the Inputs page, you’ll find the processing for your c.t. input. As the first step, just put a × -1 (under ‘Calibration’) to change the sign of the power.

Thanks, Robert that’s great thanks for taking the time to explain it.

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