[ianburrell]

Most rooftop solar can't produce power without the grid. They use cheap inverters that synchronize frequency from the grid. They also don't produce enough current to handle the peak loads from large appliances starting. The battery provides the current peaks. It is possible to buy more expensive off-grid inverters.

[JoeAltmaier]

Yup. Stanford University discovered this to their dismay. Power grid went out, even with a field full of solar panels they had no electricity until the grid came back.