So far, it really hasn't. The media has spread the perception that the whole state is completely in the dark. Of course, that isn't true. At this time, we know of no meetings or individual visitors who have cancelled their San Francisco reservations because of energy concerns.
Most hotels have backup generators.
The Moscone Convention Center is currently not subject to blackouts and also has backup generators. Those little cable cars will keep climbing halfway to the stars. In the event of a blackout, different pockets of the region are affected at different times so only a small portion of the city may lose power at any one time. Blackouts can occur when there is a shortage of electricity available so they typically happen during the day when the demand for power is higher. There are many things that visitors can enjoy without the use of electricity.