Hi, I’m new here but have been using the IoTaWatt for about 2 years now without any issues. (Runs rock solid 
)
In a few days solar panels are going up on the roof and I don’t know how and where to measure what they export. The 3-phase inverter output is on another panel that is far away from the main panel where my IoTaWatt is installed this panel has a direct line to my main panel but on that same line are also other devices that consume power (ie. devices connected to that panel).
You are asking how to measure what is exported. Most folks define export as the excess energy that is “exported” to the grid. That is, the generated power net of what you use. In other words, if your panels generate 60 kWh on a given day and your household uses 37 kWh, then you export 23 kWh. That is measured at the mains as negative power.
You probably already have CTs on your mains so if you simply check the “allow negative power” box for each of the mains inputs, you will be measuring exported power. I would suggest that you review the docs and create an integrator to track total import (.pos) and export (.neg).
Now, I suspect you are also interested in knowing exactly how much energy your solar array produces, and how much your household is using. That is a problem with the sub-panel arrangement that you describe. Without monitoring the inverter output directly with CT extensions if possible, you will only be able to measure the net production, that is the inverter production minus the sub-panel loads. You will still be able to monitor measure the loads in the main panel by subtracting the sub-panel load (with “allow negative values” on those CTs) from the mains.