Power shortages have been reported in California, Missouri and New York, with the mercury hitting an unprecedented 48 Celsius (119 Fahrenheit) over the weekend in Woodland Hills, part of the sprawling Los Angeles metropolitan region.
Authorities were investigating at least 53 deaths possibly caused by the heat, most in the steamy Central Valley.
Officials said most of the victims were elderly.
Thousands of farm animals also were dying in the heat, officials said.
The heat knocked out the Los Angeles-based servers for the hugely popular personal web page host MySpace for several hours on Monday.
"Due to the record-breaking heat in Los Angeles over the weekend, the area where MySpace's servers are stored had massive power outages and the backup generators failed," the company said in a statement. Power was restored and the network was up and running again, it said.
In the central state of Missouri, 159,000 people remained without electricity amid baking heat and humidity in the wake of two thunderstorms that hit the region over the past week that shut down power supplies.
"Due to the record-breaking heat in Los Angeles over the weekend, the area where MySpace's servers are stored had massive power outages and the backup generators failed"
A statement by MySpace website
And in New York, where at least 6,000 people in the borough of Queens have been without electricity for nine days, an antiquated power grid has provoked a bitter reaction among city residents.
The Los Angeles Times reported 13 heat-related deaths in the California Central Valley and four in southern California. State authorities also were investigating the death of a 79-year-old woman in Stockton, the Sacramento Bee reported.
California was facing possible brownouts as the average temperature in Los Angeles hovered near 38 Celsius (100 F) over the past two weeks, pushing up the use of air conditioners and taking nearly all state's electricity generation capacity.
Electric power demand hit a record 50,538 megawatts on Monday and a similar spike in energy demand was expected on Tuesday.
Amid fears that the electricity network would be overwhelmed, authorities issued a "stage two" alert on Monday that allows for power cuts to businesses that previously agreed to scheduled blackouts.
Cross country blackout feared
If demand continues to mushroom, officials will invoke a "stage three" alert that would trigger rolling blackouts across the country's most populous state.
Power lines have been broken
along several highways in the US
Some 50,000 households in California were without electricity on Sunday, including parts of San Francisco. Since July 13, more than 700,000 households have experienced power cuts at some point, the Los Angeles Times reported.
California has lived under the threat of power cuts since a crisis in 2001 that was blamed in part on deregulation of the electricity sector. The power shortages forced the state to declare a state of emergency.
Some parts of the electricity network date back to the 1930s and officials in Los Angeles acknowledge that the strain on power supply stems from a lack of investment in public infrastructure stretching back for years.
California officials pinned their hopes on a weather forecast that anticipated the heat would subside in the Pacific coast state on Wednesday.
Sizzling New York
On the other side of the country, thousands of New Yorkers were also still without power in the ninth day of a blackout, with no clear estimate of when all the services might be restored.
"It's surreal. This is New York City and it shouldn't be this way with the taxes and utility bills we pay"
Peter Hidasi, a Queens resident
The worst of the heat this week has been in the West, where California air conditioners drove electricity use to 50,270 megawatts on Monday - a state record.
Close to 100,000 people in the city were without power over the weekend one week after city power supplier Consolidated Edison was hit by breakdowns.
Some streets were taken over by refugees from the blackouts, with the Red Cross providing food, water and ice to people seeking relief from the heat. Doctors organised sidewalk consultations.
"It's surreal. This is New York City and it shouldn't be this way with the taxes and utility bills we pay", Peter Hidasi, a Queens resident, told the newspaper USA Today.
Some 6,000 people and 750 businesses were affected by power cuts on Tuesday, including restaurants and ice cream shops.
In Missouri, more than a quarter of the 600,000 people who lost power in the wake of huge storms last week remained without power, most of them in the city of St Louis.