Sweet Answer from Solar Mango: (updated Jul 2015)
Yes, a grid connected rooftop solar power plant will not work unless there is a backup power source such as a battery or diesel generator connected in the system.
In countries where grid power is not available 24X7, many prospects of rooftop solar power plant are shocked when they hear that the rooftop solar panels will not generate electricity when there is a power shut down. They often wonder aloud,”So, what is the point?”.
Why does the grid-connected solar power plant not generate power when the grid is down?
As the amount of sunlight falling on the panels varies during the day (due to clouds, etc.), the power output from the panels also varies. As this variation could damage equipment that is powered by solar, the inverter continuously matches the PV plant’s output to another source of steady power. Therefore a rooftop solar PV that generates AC power will always needs another source of power (whether the grid or diesel generator or batteries) to provide a reference voltage in order to function.
If the inverter is designed to use only grid power as a reference voltage, the plant will not generate power when the grid is down even if there is ample sunlight.
There’s another reason why solar power plants do not generate electricity during a grid failure. When a power failure occurs, a portion of the grid isn’t energised. This non-energised portion of the grid is known as an island. If the solar plant is pumping electricity into this non-energised portion of the grid during a power failure, it might cause utility personnel who are working on the grid to be electrocuted. To eliminate this risk, the inverter in the solar power system turns off the power from the plant.
Note however that solar panels can continue generating electricity if such a grid connected system also has a backup power source such as a battery or a diesel generator set. This is so because, even in the absence of grid power, the backup power source provides the necessary reference voltage.
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