Photovoltaic system, heat pump, household electricity, and three bi-directional meters

I know cascade wiring with 2 meters. I assume the ZE is not an official meter, but purely to satisfy the owner's curiosity about how much electricity was produced

They are all meters of the local network operator (and accordingly "sealed" - completely official). Might have to call the city utilities next week to see if they can shed some light on it.

Reads like measurement concept 8 with third meter (ZE), so that you don't have to calculate yourself.

Sounds reasonable. But why is the third meter then sealed (and chargeable?). Wouldn't a privately installed meter be sufficient?

Would be, but isn’t. In principle, you could also just calculate what he shows.

This is what it looks like. If the VNB specifies it this way, then you have to pay.