Solar systems generate electricity once the sunlight hits the panels. This electricity then passes through an inverter, where it is converted to alternating current (AC) for domestic use. If you have more power than you need, the excess is added into your power grid.
When severe weather happens and the lights go out utility experts are usually busy trying to restore power under the assumption that the lines are dead. To ensure these experts are not hurt, all Solar Only Systems are designed to shut down once there is a power outage.