You need more hardware when you want to sync to grid after being off grid. You need one on the inverter and a seperated output, most grid-tie inverters simply have one port for everything. If you use one port for everything you can hardly sync the house grid to the external grid since you can't both provide power and try to sync the phase.
Err, you can provide the power, you do need another voltage sensing connection to the grid though, and might want an automated breaker to not have to attend the device and re-connect inverter and grid once sync is complete. If the inverter is properly fused, you should even get away with a LED or a small display that shows you whether it's safe to reconnect or even how long until sync is complete.
