# Shelly EM Integration

I’m confused as to how to use the Shelly EM properly to monitor the entire home.

I’m using the Shelly integration, which I think is fairly new? Curious if there’s a way for the UI to calculate and display the actual power draw with two CT sensors on a North American split-phase power system, if I’m asking the right question.

I’m looking to monitor the power draw for the entire home (240v split phase.) I have the CT clamp on the two incoming Line wires. This means the display results in one negative and one positive reading. But the difference between the two is not the actual usage! I have a Blue Line Innovations monitor that reads the hydro meter infrared blips, and I get around 250w with the home “idling” (no appliances, minimal lights, server rack on.)!

The vast majority of install and reviews on the internet are for European single phase 240v where they only use one CT for the entire home.

If you are using the typical US system of either end of a centre tapped transformer then the lines are 180 degrees out of phase. So the total power used is just `abs(P1) + abs(P2).` Your voltage reference (powering the device) should be from one of the lines to neutral (120V). Not across the two lines (240V). This will measure power from both lines to neutral and from line to line.

If however you are using two phases of a three phase system (120V line to neutral, 210V line to line) then the lines are 120 degrees out of phase and the powers have to be added like so:

`abs(sqrt(3)*P1) + abs(sqrt(3)*P2)`

Again, your voltage reference (powering the device) should be from one of the lines to neutral (120V).

I just got confirmation that the Shelly EM is calibrated to use the CT clamps that they sell in their store. Unfortunately they’re too small to fit around the mains in my hydro panel (50A and 120A are the same size) so I bought the standard blue 120A clamps… but calibration is wrong so the Shelly readings are wrong, making the results wrong in Home Assistant.

You should be able to calibrate them with a template sensor.

Just to show an example of the calibration error using the Shelly-provided white CT clamp by Allterco (Channel 1) and the YHDC brand commonly found on the internet (Channel 2.) I simply put both clamps on the same main line coming into the hydro panel.
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Another example with other energy monitors: