The grid tie inverter can be plugged in, as it only will supply power when it detects there is a supply present from the mains. Thus if you disconnect the plug it will detect no load and switch off until it is getting power again from the mains.
It simply takes the power from the solar panels and uses it to assist the mains supply, reducing the current drawn from the mains supply to supply the loads in the house, or feeding power back if there is little load in the house. There is no control of power aside from it doing the best it can with the solar power available. No mains means no power converted and no output.
There are grid tie systems with battery backup, but these are both not common and are limited in capability, you typically will either have grid tie only or solar and battery with an inverter to supply loads in the house. There are systems coming out that combine them by charging the battery when there is solar power present then feeding the excess power with a full charge back into the grid.
