So,

I finally set up my configuration with one WEM3080TD to measure the grid, and one WEM3080 to monitor the PV inverter. Note the "D", it is a "Delta" 3-phase grid with no neutral. The Inverter (Fronius IG40) AC output in single phase. It looks like this:

The kWh measurements I get on the PV side seem correct, they are similar to what I see on the inverter display. I am wondering if something is wrong on the grid side: Here is a typical example when the the sun is shining and the panels produce electricity. Note that at the moment of this snapshot, there is no one at home and electricity consumption is rather limited (fridge, set top box, PC in sleep mode, ... but no heat pump or other power device):

No way there are currently devices running that sum up to nearly 8kW. The PV production delivers 4 kW, which makes sense, I have 4.5 kWe total installed. The negative feed-in, also 4 kW, makes also sense, almost all production is injected to the grid. But why is it negative? I double checked the clamps, which are on the wires right after they exit the utility-provided breaker and before they enter the cabinet and arrive at a another breaker. They are placed with the arrows in the direction of the house.
My understanding is that negative feed-in means importing from grid. Shouldn't that feed-in be positive?

Hi:

If there is nearly no electricity consumption at that moment but the grid power display -3891w, it must be that the CT that clamps on the wire in the wrong direction.
The grid power ought to be 3891w, not -3891w.
Then the load power would be 

solar production power - feed in power

load power  =3923w-3891w=32w

Please  clamp the CTa and CTc in another direction