I've seen a few messages forwarded from friends at various SF companies, especially South of Market, saying that PG&E has been calling them and asking that they reduce power utilization.

The California Independent System Operator (runs the electrical grid) has issued a stage one alert. A stage one alert is to inform the public of the potential for power outages, and request consumers curtail their power consumption. Technically a stage one alert means the electrical grid reserve is less than 7%. Stage two is declared when the reserve drops below 5%, and large commercial customers with interruptible power contracts will be requested to curtail their load. Stage 3 is declared when the reserve drops below 1.5%, involuntary power interruptions (aka rolling brownouts and blackouts) are probable at stage three. CAISO is looking at the statewide load when issuing its alerts. Individual areas may be better or worse off. At 2pm San Francisco Office of Emergency Services has issued this statement:

The Pacific Gas and Electric Company has notified the City of San Francisco that PG&E will begin involuntary curtailment of electrical service to portions of San Francisco. These outages are necessary to prevent collapse of the electrical power grid. Parts of San Francisco are expected to be without power for at least an hour to an hour and a half.