The operator of New England’s power grid has extended its request for electricity conservation measures Thursday as demand for power is expected to peak at near-record levels.

ISO New England warned earlier this week that the prolonged combination of heat and high humidity throughout the region could push energy demand close to capacity, possibly requiring the importation of power from other parts of the country. On Thursday, it extended the request for voluntary conservation measures, such as raising air conditioning temperatures to 78 degrees.

Preliminary data indicate that electricity demand peaked Wednesday at 26,655 megawatts, which is the 10th-highest demand day on record in New England. Demand peaked at 26,840 Thursday, according to ISO New England’s website, and the demand was expected to remain high on Friday.

New England’s record for electricity use was set on Aug. 2, 2006, with a peak of 28,130 megawatts. One megawatt of electricity can power about 1,000 homes in New England.

To watch power demand rise in real time, go to the ISO New England website.

The temperature is expected to reach 87 degrees in Portland on Thursday afternoon, according to the National Weather Service. It’s expected to reach 89 degrees Friday and 87 degrees Saturday, before cooling off and dropping back into the 70s on Sunday.

Portland’s high temperatures have ranged between the mid-80s and low 90s since Sunday.

The same stretch of hot weather throughout New England is testing the regional power supply, and demand goes up the longer a heat wave lasts because it takes more power each day to keep buildings cool.

The grid operator is targeting its request for noon to 8 p.m., when demand for power is greatest. It suggests these measures, when they don’t interfere with health or safety:

• Raise air conditioning temperatures to 74-78 degrees.

• Turn off air conditioners when leaving home for an extended period.

• Turn off unneeded lights, appliances and office equipment.

• Wait until early morning or late evening to do laundry or other chores that use a lot of electricity.