Most home solar installations are set up such that if the grid is down, the solar system is down. They are rigged this way to prevent the solar installation from back-feeding power into the grid and electrocuting linemen.
Some more sophisticated systems have a grid isolation switch which allows the homeowner to get power from the solar system without backfeeding power into the grid.
I don't know why all solar installations typically do not come with this isolation switch. My brother in law's system has one and whole house generators have them.