How California's Power Crisis Works

By: Kevin Bonsor

Rolling Blackouts

When California's power supply dips, the California Independent System Operator (Cal-ISO), who manages the state's power grid, notifies the California utilities that there must be a load reduction on the statewide power system. The individual utilities than determine how the load reduction will be accomplished. Usually, it's done by blacking out certain blocks in their area for hours at a time.

California ISO is an independent agency charged with managing the flow of electricity along the long-distance, high-voltage power lines that make up the bulk of California's transmissions systems. It also has the task of safeguarding the reliable delivery of electricity. The Cal-ISO does not, as some reports have stated, order black outs. Only local electric suppliers have the ability to do that.


Rolling blackouts are typically used only in severe cases, and are designed to prevent a complete collapse of the state's power system. It signals that the state's operating reserves have fallen below 1.5 percent.

Here's how California's rolling blackouts work:

  • The state is divided into large blocks by the various utility companies. For example, PG&E divides their service area into a number of blocks.
  • When an energy crisis reaches a Stage 3 emergency, California ISO notifies local electric suppliers that there will be a load reduction on the statewide system. These local suppliers than implement a system of rotating power outages.
  • These outages proceed in numerical order, beginning with block number one. If the crisis continues, number two will be the next block affected by the rolling blackouts. PG&E customers can find their block number on their service bill.
  • Hospitals, police stations, fire departments and some residents located near these emergency agencies are unaffected by the rolling blackouts.