Californians did an impressive job conserving energy Sunday, but power regulators are forecasting record energy demands as everyone heads back to work today, making conservation absolutely critical.

"While we got through today just fine, we're very concerned about Monday. Everybody goes back to work and cranks on the computer, the coffee machine, the fax machine," said Gregg Fishman, a spokesman for the California Independent System Operator, a nonprofit corporation that manages the state's power grid.

"A lot of the office buildings that closed over the weekend have heated up, so the air conditioning load on a hot Monday is often incredible," Fishman said.

The state likely will hit a Stage 2 power emergency today and could face rolling blackouts, a Stage 3 emergency, in the afternoon, when usage levels typically peak, Fishman said.

An estimated 436,000 customers in the greater Bay Area lost power at some point Saturday or Sunday in heat-related power failures, PG&E spokesman David Eisenhower said. By 8 p.m. Sunday, service had been restored to 73 percent of those customers, leaving about 124,000 without power. Eisenhower said Bay Area crews were responding to 2,400 separate blackouts, most in the South Bay and inland East Bay.

Conditions are expected to be ideal -- heat, humidity and little to no wind -- for creating smog. So the Bay Area Air Quality Management District has declared the season's ninth Spare the Air day, when commuters are urged to carpool or take mass transit. Buses, trains and ferries will not be free today, however, as they were on the six Spare the Air days that fell during the workweek, because funds have run out.

Even as temperatures dropped slightly Sunday, records were broken across the Bay Area.

The high of 104 Sunday in San Jose beat the record for the date of 97 set in 2005. San Rafael's 106 surpassed that city's record 99, also set last year, while the Associated Press reported the temperature reached 113 in Livermore. And San Francisco reached 83, barely topping the 2005 record of 82.

Oakland's high of 89 fell short of its 2005 record of 92.

Meteorologists blamed a stubborn high-pressure system that heats air and destroys clouds.

"It's been fairly stationary over the last several days, and it's been able to get to an atypical strength," said Brooke Bingaman, a National Weather Service meteorologist. "That's Mother Nature. She keeps us guessing and throws us curve balls every once in a while."

At the New York Frankfurter hotdog stand at Powell and Market streets in San Francisco, business was steady but different Sunday. Joe McKeon, who has been in the hotdog business for 12 years, said he was selling a lot more drinks and ice cream than dogs.

Tourists' questions were a little different, too.

"Usually around this time of year, I get freezing tourists coming straight from SFO wondering where they can buy a long-sleeve sweatshirt," McKeon said. "I didn't get that question today."

Around the corner from McKeon's umbrella-shaded stand, San Francisco street-cleaner Israel J. Taylor said he spent the afternoon picking up a lot more trash than he does on cooler days.

"It seems like when the temperatures elevate, so do people's attitudes," said Taylor. "Today I've seen a lot of people throw their garbage right on the ground even if a trash can is right in front of them. I guess when it's this hot, walking 2 feet is a long way for some people."

The first six months of 2006 have been the hottest on record for the United States as a whole, according to the National Climatic Data Center, which has collected data since 1895.

"Whether or not that's global warming, everyone has an opinion," Bingaman said. "You can find statistics for it and against it."

Bingaman forecast the high-pressure system to weaken and move out by next weekend, when temperatures should return to normal.

Consumers have already cut energy use significantly. Demand for electricity was down by 200 megawatts from Saturday -- the equivalent of eliminating 140,000 homes, according to PG&E.

But that wasn't enough. The power company is urging people to set thermostats to at least 78 degrees today and cut back on using dishwashers and other large appliances.

A Stockton nursing home patient died Sunday when the air conditioning failed at the Beverly Healthcare Center, a spokeswoman said. Staff at the center transferred 103 patients to nearby nursing homes at 4:30 a.m., about an hour after the cooling system shut down, said spokeswoman Amy Knapp. The patient died after being taken to a local hospital.

In Sunnyvale, the heat and a power outage at a mobile home park prompted city officials and the Red Cross to open a relief center at Columbia Middle School. Residents of the mobile home park, which has about 1,000 homes, could spend several hours and possibly the night in the school's air-conditioned gym.

The Associated Press reported four other deaths across the state this week may have been caused by the heat, and 50,000 Southern California homes were without power Sunday evening.

Though today will be this year's ninth Spare the Air day, including Saturday and Sunday, only one was declared last year, said Luna Salaver, a spokeswoman for the Bay Area Air Quality Management District.

"The budget for six free fare days has been exhausted," Salaver said. "We were totally caught off guard to use up all these days before the season ended. To run out so quickly was an experience no one predicted."

The Metropolitan Transportation Commission, a regional agency, originally set aside $7.5 million for three days of free rides. After hot, windless weather in June exhausted that money, the commission came up with $5.3 million to fund three more days of rides.

Salaver suggested biking, carpooling and taking public transit -- even if it's not free.