A Solar PV system is capable of producing more power than your property uses in a day, this excess goes back to the grid. During a power outage you cannot export power to the grid as it could injure line-workers so all grid-tied Solar PV systems are designed and required to shut down automatically as soon as they stop detecting the grid. This is called "anti-islanding". Additional components can be installed that allow a part or all of a grid-tied solar PV system to operate during an outage but this typically requires a significant investment and is not trivial.