By Scott DiSavino
(Reuters) - The biggest electricity system in the nation said it does not expect supply problems on Friday or over the weekend despite a continuing brutal heat wave.
PJM, which operates the power grid in the mid-Atlantic and Midwest United States, said in a news release on Friday that it will keep a hot weather alert in effect for Friday and Saturday.
A hot weather alert means that transmission and generation operators should defer maintenance on generators or power lines and be prepared for above-average demand for electricity. This is normal given forecasts for temperatures in the 90 degrees Fahrenheit (32 Celsius) range, PJM said.
High temperatures were expected to reach 100 degrees F in Chicago, 97 in Philadelphia and 101 in Baltimore, the biggest cities in PJM, according to AccuWeather.com.
PJM forecast peak demand Friday would reach about 150,000 megawatts (MW) after hitting a preliminary peak of 148,392 MW on Thursday.
That would still be well below the grid's all-time peak of 163,760 MW set during a brutal heat wave in July 2011. Before the summer, PJM had forecast peak demand this summer would reach 153,780 MW.
One megawatt can power about 1,000 homes.
Even though the grid is managing through the heat wave, there are at least 400,000 homes and businesses still without power in Ohio, West Virginia, Maryland and Virginia following a series of violent thunderstorms last weekend.
Some homes and businesses in those states have not had power for a week and may not get it back until early next week.
High temperatures in Charleston, West Virginia, the hardest- hit city in the hardest hit state, were expected to reach 98 degrees F on Friday, 101 on Saturday and 95 on Sunday before easing to more normal levels in the 80s on Monday.
PJM said it does not expect any problems over the weekend, even though hotter temperatures are forecast to move into the heavily populated East Coast cities, because power demand will be lower with most businesses closed on Saturday and Sunday. It expects peak demand on Saturday in the low 140,000 MW range.
Saturday temperatures were forecast to reach the triple digits for Philadelphia and Baltimore and the 90s F in Chicago. The heat wave is expected to move out to sea by Monday, when temperatures across PJM will return to near-normal levels in the 80s, AccuWeather.com said.
PJM serves more than 60 million people in all or parts of 13 states and the District of Columbia.
The biggest power companies in PJM include units of Duke Energy, Exelon Corp, FirstEnergy Corp, American Electric Power, Dominion Resources Inc and Public Service Enterprise Group Inc.
(Reporting By Scott DiSavino; editing by Jim Marshall)