OpenEnergyMonitor Community

ACAC sensor on a UPS

Hello, I’ve just ordered an emonPI shield kit with an ACAC Sensor.
I’ve only one power outlet where I want to install my setup, with an UPS plugged in.
Does the ACAC sensor works with the UPS between it and the power outlet ? Or i should by a multisocket adapter ?
Thank you

Hello @aymeric106, It’s probably best to have it plugged directly to a power outlet, it depends on whether the UPS will modify the voltage waveform in any way when it is running in off the mains rather than backup mode?

What is the UPS supplying? If it is supplying appliances and equipment whose power consumption you want to measure, then you should have the a.c. adapter on the output side of the UPS. If the UPS is supplying only a computer (emonPi) running emonCMS, then you should have the a.c. adapter on the mains side of the UPS.

The a.c. adapter should work on the output side, but as Trystan says, the output of the UPS will almost certainly not be a good sine wave like you get directly from the mains electricity supply. If the a.c. adapter can faithfully transfer the shape of the UPS output, then you will accurately measure the power on the output side. On the other hand, if you have it on the mains (input) side of the UPS, the voltage it measures is not likely to represent the voltage on the output side.

What I do not know - because I do not have your UPS to test it with - is whether the a.c. adapter will give you an accurate picture of the mains wave.

So either way, something will not be right.

Thank you, i will plug the ACAC outside the UPS then, and the raspberry power behind the UPS.

I’m not sure I understand exactly what it used for, but since I want to monitor my house overall energy consumption, I guess it needs to raw waveform, not the one filtered by the UPS.
I thought maybe it needed to be exactly on the same circuit as the raspberry power input.


You want to measure the power in a circuit. The a.c. adapter measures the voltage in that circuit, the current transformer measures the current. Your emonPi (or whatever you use) takes samples of the current and voltage and calculates the power.

As you want to measure your total energy consumption, you must measure the voltage (that is, plug in the a.c. adapter) as close as possible to where the power comes in, and attach the current transformer(s) before your distribution board where the wires split to feed individual floor or rooms or appliances.

Do not plug in the a.c. adapter on a circuit where there is a high power appliance, because when that operates, it will lower the voltage and give a wrong reading for the rest of the house. The rules in the UK allow a “voltage drop” of 2.5%, so that would be the error in your readings if you measured the voltage in the wrong place.