U see. You connect to 2 floor db. So the energy source come from 2nd floor. It’s not even direct to Tnb.

So ur 2nd floor will use the energy first. Then extra go back to main db. And then get istributed to the appliance.  Then extra flow back to Tnb.

But doesn’t mean ur ground floor will use Tnb energy first. You still use solar for self consumption first before any extra is send out.

When cloudy day, not enough producing , that’s when Tnb kicks in.  That’s the way how on grid inverter works.