Grid operator warns of rolling blackouts.

California found itself in danger of another round of rolling blackouts in the early evening hours of Saturday, amid the start of a broiling heat wave expected to last through the Labor Day weekend.

The California Independent System Operator, which oversees the grid for about 80 percent of the state, at 6:30 p.m. issued a Stage 2 Emergency — which means it is no longer able to provide its expected energy requirements. At Stage 2, the grid operator resorts to interventions such as ordering power plants online.

Just a few minutes before issuing the Stage 2 Emergency, CAISO officials tweeted that fires in areas of the state caused a generator and a solar farm to trip offline. “We need everyone to reduce #electricity use at this critical time,” the grid operator’s Twitter feed said.

The next and final level of emergency is a Stage 3 Emergency,

in which the CAISO orders utilities across the state to shut off power to make sure energy demand does not outstrip supply, thus shutting down the grid.

Just hours before the Stage 2 Emergency was declared, some 18,500 customers in areas across San Diego County lost electricity in the afternoon hours.

A spokeswoman for SDG&E said about 1,000 customers lost power because a fast-moving brush fire in East County impacted the utility’s equipment. Dubbed the Valley fire, the blaze burned 1,500 acres in the Japatul Valley area by early evening.

But another 17,500 customers lost electricity in areas unaffected by the Valley fire, ranging from neighborhoods near downtown San Diego such as Golden Hill to inland areas already suffering triple-digit high temperatures such as El Cajon and La Mesa.

SDG&E officials were still assessing what led to the afternoon outages, saying that “some are weather related.”

According to the utility’s outage map, some areas lost power around 2 p.m. and the outages then spread to other areas for the next four hours.

For example, a circuit in Lemon Grove went down at 5:07 p.m., affecting 1,368 customers.

As of 6:30 p.m., the outage map recorded 40 circuits out of commission. Some were expected to come back online within a couple of hours but others had expected restorations as late as 9:30 p.m. The downed circuit with most affected customers was located in the Valley Center-lilac area, with 1,508. That circuit was expected to come back into service at 9:30 p.m.

A circuit in East Ramona and San Diego Country Estates knocked out power to 1,330 customers. It was estimated to go back online by 9:30 p.m.