RALEIGH -- As rising temperatures continue to affect much of the country, two North Carolina-based utilities are cycling off thousands of household air conditioners to conserve energy during the oppressive summer heat.

Progress Energy on Friday briefly shut off household air conditioners for 63,000 customers in the Carolinas during a three-hour period, the News & Observer of Raleigh reported Saturday, marking the fourth time since May the Raleigh-based utility has relied on its residential customers to ease the strain on the utility's power grid.

Charlotte-based Duke Energy briefly shut off household units on two afternoons this past week for more than 170,000 Carolinas customers.

Participating customers, many of whom are at work when the programs are activated, aren't told ahead of time when their units are cut off and on, but they are paid to voluntarily let the companies cycle their power on and off. Progress' EnergyWise program pays customers $25 a year to participate. Duke's PowerManager comes with an annual $32 thank you, but customers have to pay an initial $35 fee.

"We're trying to minimize the discomfort as much as possible so that customers will want to remain with the program," Progress spokesman Mike Hughes said. "It doesn't do anyone any good to shut off air conditioners for three hours ... and then have thousands of customers decide it's not worth it to participate in the program."

Over the course of a hot afternoon, as thousands of air conditioning units are cycled on and off, Progress and Duke can shave off enough power to offset a small power plant -- usually an older, dirtier coal-burning unit that doesn't have to be fired up.

Duke said its program costs $6.8 million a year, while Progress' costs $4.5 million.

The feature could one day be standard for all customers when "smart meters," equipped with two-way communications, are the industry norm.