Can photovoltaic systems usually operate in grid-connected mode and still manage critical loads when utility service is down?
A: Yes, however, storage battery must be used. This system is extremely popular for small businesses and homeowners where a backup power supply is required for critical loads such as refrigeration, lighting, water pumps, and other necessities. Under normal circumstances, the system operates in the grid-connected mode, serving the on-site loads or supplying excess power back to the grid while keeping the battery fully charged. When the grid is failed, the control circuit in the inverter operates the inverter from the battery to supply power only to the dedicated loads. In this configuration, the critical loads must be provided from a dedicated sub panel. Figure bellow shows how a PV system might be configured to operate generally in the grid-connected mode and also power loads from a storage battery when the grid is failed.