A Grid-Tied system works by supplementing the national grid. Power generated by the solar panels is used to provide power to the load. If the load exceeds the power that can be provided by the system, grid power is used to supplement the remaining power needed.
The inverter is used to convert the DC power from the solar panels to AC power. A Grid-Tied system will NOT work when grid power is off because the system needs to be connected to the grid to function. There will also be no backup if the power is out because normally no batteries are used in these kinds of systems.