By Steve Gorman and Nichola Groom

LOS ANGELES (Reuters) - California's power grid operators warned homes and businesses on Monday to conserve electricity as rising demand for air conditioning stoked by a record-setting heat wave across the U.S. Southwest tested the region's generating capacity.

The so-called Flex Alert was posted until 9 p.m. Pacific time during a second day of triple-digit temperatures that strained Southern California's energy production, creating a potential for rolling blackouts on the first official day of summer.

But the peak hour for energy demand came and went Monday evening without disruption of the region's power delivery network, the California Independent System Operator (ISO) reported.

"Since we're past that and have not experienced any trouble, I think we're headed into the safe zone," agency spokeswoman Anne Gonzales told Reuters.

Temperatures were expected to begin abating on Tuesday, according to weather forecasts. As of Monday night, there were no plans to extend the Flex Alert, ISO officials said.

Monday's alert was the first big test of power generators' ability to meet heightened energy demands in the greater Los Angeles area without natural gas supplies normally furnished by the now-crippled Aliso Canyon gas storage field, effectively idled since a major well rupture there last fall.

The oven-like heat prompted the city of Los Angeles to keep its network of public "cooling centers" - libraries, recreation centers and senior centers - open for extended hours as a haven for people whose homes lack air conditioning.

Area home improvement and hardware merchants were doing a brisk business in fans and AC window units.

Brett Lopes, 31, a freelance lighting technician, stopped in a Home Depot outlet near downtown to buy supplies for a homemade air conditioner he called a "swamp cooler" to use while he waited for his landlord to repair his broken AC unit.

"It's brutal," he said of the heat, explaining that he looked up directions on YouTube for assembling the makeshift cooling device. "It doesn't work as well as AC, but it's better than sitting in 100 degrees."

Others flocked to public swimming pools.

"It was really refreshing today, but more crowded than usual," said Paul Stephens, 31, a pastor who was swimming laps at the Rose Bowl Aquatic Center in Pasadena, where the mercury climbed to 108 Fahrenheit (42 Celsius) .

BALANCING THE GRID

The ISO, which runs the state's power grid, urged consumers on Monday to cut back on electricity usage, especially during late-afternoon hours.

Utility customers were advised to turn off unnecessary lights, set air conditioners to 78 degrees Fahrenheit or higher, and wait until after 9 p.m. to run major appliances, such as clothes washers and dryers.

Gonzales credited public cooperation with the flex alert for likely helping avert widespread outages on Monday.

Large stretches of three states sweltered in a second straight day of record, triple-digit temperatures, as the National Weather Service posted excessive-heat warnings through Wednesday for southern portions of California, Arizona and Nevada, though the hot spell appeared to have peaked on Monday.

Power customers ranging from homes and hospitals to oil refineries and airports are at risk of losing energy at some point this summer because a majority of electric-generating stations in California use gas as their primary fuel.

Since the energy crisis of 2000-2001, the ISO has imposed brief, rotating outages in 2004, 2005, 2010 and 2015, mostly related to unexpected transmission line or power plant failures during periods of unusually high demand.

With California's largest natural gas storage field shut down indefinitely at Aliso Canyon, state regulators have warned that Los Angeles faces up to 14 days of gas shortages severe enough to trigger blackouts this summer.

Aliso Canyon, owned by Southern California Gas Co, a division of San Diego-based utility giant Sempra Energy, normally supplies the region's 17 gas-fired power plants, hospitals, refineries and other key parts of California's economy, including 21 million residents.

The gas leak there, ranking as the worst-ever accidental methane release in the United States, forced thousands of nearby residents from their homes for several months after it was detected last October. The leak was finally plugged in February.

(Reporting by Scott DiSavino in New York; Writing and additional reporting by Steve Gorman in Los Angeles; Editing by Leslie Adler and Andrew Hay)