Demand heats up

Power plants had been alerted the day before, and made preparations to deliver the roughly 27,000 megawatts the state’s 6.5 million households and businesses were expected to use yesterday as they switched on air conditioners and fans.

But last night outages were beginning to show up in areas across the state.

Mark Reed, a spokesman for NStar Electric, said last night that 4,000 customers were without power as of 11 p.m. in scattered pockets throughout its region, which extends from Burlington to Marshfield, and as far west as Dover.

In Boston, he said, 1,300 customers were without power in various neighborhoods. Residents in Jamaica Plain and Brighton had reported outages.

Reed said the outages can be attributed to demands from the hot temperatures, but that NStar would be investigating further in coming days.

He could not say when power would be restored.

About 50 communities powered by National Grid were without power as of 11 last night, according to the company’s website. The 90 power outages were affecting 2,576 National Grid customers, the website reported.

National Grid spokeswoman Jackie Barry said a lot of the outages during the day took place in the Merrimack Valley, and one outage in Lowell affected about 3,000 customers. Barry said with the exception of a few outages, the majority of the outages were small, scattered, and caused by the heat.

“The system can withstand significant heat, but today was pretty excessive and yesterday was pretty excessive,’’ she said. “It’s not surprising that we had scattered outages today, given this kind of weather.’’

Kristin Anderson, a spokeswoman for Unitil Corp., said the utility had no outages as a result of the heat.

Though hot, officials earlier yesterday said the weather was not expected to overload the power grid, which can provide roughly 32,000 megawatts.

On an average summer day, electricity use runs from 19,000 to 25,000 megawatts, according to the region’s grid operator, ISO New England.

“We like to say we work year-round to prepare for the summer heat, because this is when our system is pushed to its limit,’’ said Caroline Allen, a spokeswoman for NStar, a utility company based in Boston that serves 1.1 million Massachusetts customers.

Throughout the day, engineers worked side by side with dispatchers at NStar’s command center to try and make sure no single circuit moved too much power.

Many NStar construction crews also postponed scheduled work, remaining at the ready to respond to any heat-related issues.

But power issues popped up, especially in Boston, where several manholes caught fire in South Boston as a result of demands from the heat.

The Fire Department responded to 10 calls between 6 and 10 p.m. for power lines on fire on utility poles. The fires broke out in Jamaica Plain, Roxbury, Brighton, Dorchester, and Charlestown, said Steve MacDonald, a department spokesman.

In South Boston, a manhole exploded, spreading fire to a second manhole.

Fire officials said the fires were the result of power overload, from residents and businesses trying to beat the heat.

“It’s overuse. There’s a draw on the system,’’ said Boston Fire Department Acting District Chief Norman Earnest, at the scene of the South Boston explosion.

“People are home running their air conditioners. Of course, I’d be doing the same thing.’’

MacDonald said that NStar was unable to provide firefighters an estimated wait time for a utility crew to be on scene at most of the calls.

“They’re just too busy. They’re getting to them, but they’re pretty busy tonight,’’ MacDonald said.

At EnerNOC, a Boston energy-management company, Don Jenkins and his colleagues monitored maps of weather patterns and energy use while waiting for ISO New England to call for their services.

At any given time, EnerNOC, which pays its customers to reduce their energy use, can decrease power use in various parts of the country by up to about 4,350 megawatts.

“We’re sitting here, hovering our finger over the button ready to push and go,’’ said Jenkins, who is senior manager of event operations and data analytics at EnerNOC.

In the morning, the company sent e-mails to 8,000 contacts, he said.

Employees kept in constant touch with customers across the country, dimming lights, turning off water features, or helping buildings power down as necessary in several states with high temperatures.

At ISO New England, spokeswoman Ellen Foley said electricity use was not expected to reach the record high, set on Aug. 2, 2006, when customers used 28,130 megawatts.

The grid is also better equipped to handle larger amounts of power than in years past, Foley said. In the past five or six years, more than $4 billion has been spent to upgrade the transmission system, and $5 billion more worth of upgrades are planned.

Erin Ailworth can be reached at eailworth@globe.com. John M. Guilfoil of the Globe Staff and Globe correspondents Vivian Ho and Stewart Bishop contributed to this report.