Yes, as others post, no common grid tied system provides any backup power.
The grid tie inverter provides line voltage accuratly matched to the grid. It is required by law to automaticly shut down very quickly indeed if the grid connection is lost.

For backup power you need either a generator or a battery bank and off grid inverter, or ideally both.
The battery bank may be charged from grid power when available, from a generator, or from a PV system.

My ideal system would be a large grid tied PV system for the financial and enviromental gains, though it is useless in a power cut.
I would then install a large 12 volt battery bank with an off grid inverter for backup power.
This battery should be kept charged by its own dedicated PV module. This will keep it charged and replace any small or infrequent discharge.
After any deep or frequent discharge, this backup battery may be charged from grid or generator power.

In the event of TEOTWAWKI, then the PV modules in the now useless grid tied system could be re-purposed for battery charging.
I would advise against this for routine power cuts whilst times are normal. The grid tied array works at lethal voltages, and there are often warrenty issues involved in any modification.