Overheating transformers knocked out power to 852 SDG&E customers across four La Jolla neighborhoods Wednesday, Sept. 9, as sustained high temperatures strained the electrical grid.

The larger of two overlapping outages hit around 10:30 a.m., cutting service to 840 customers in La Jolla Heights, Hermosa, Bird Rock and a few streets in Windansea, lajolla.ca reported. A separate outage struck 12 Bird Rock customers around the same time. SDG&E estimated both would be restored by 3 p.m. Wednesday.

The 12-customer Bird Rock outage had an outsized effect on the neighborhood's commercial strip. Food service businesses shut down by 11:35 a.m., according to Beach Break Market owner Alec Hurley.

SDG&E spokesperson Anthony Wagner said the failures resulted from transformers overheating under sustained high temperatures while refrigerators and air conditioning units drew heavy loads.

"Prolonged heat definitely stresses the grid," Wagner said.

The utility reported roughly 29 outages countywide in the 24 hours before the La Jolla failures. Wagner said SDG&E had positioned repair crews around San Diego County to respond quickly. No confirmation that power was restored by the 3 p.m. estimate appeared in public records as of Thursday.

The outages came during a National Weather Service (NWS) extreme heat warning in effect through 8 p.m. Wednesday, Sept. 9. A heat advisory replaced it and was scheduled through 8 p.m. Thursday, Sept. 10. The NWS forecast highs in the 80s at San Diego beaches and into the 90s in areas of higher coastal terrain during the heat event. Fox 5 San Diego reported that San Diego hit 95 degrees on Monday, Sept. 7, breaking the city's high-temperature record for that date.

The night before the La Jolla outages, nearly 10,000 SDG&E customers across the county lost power in a separate wave of failures, Fox 5 reported.

Residents looking for relief can visit the La Jolla/Riford Library at 7555 Draper Ave., listed as a free, air-conditioned cool zone, according to NBC San Diego. The library can be reached at 858-552-1657.