DALLAS—About 15,000 customers, most of them in Dallas County, remained without electricity Saturday after severe storms pounded the North Texas area earlier in the week.

“We are optimistic we will have fully repaired the storm outages by Sunday morning,” said Catherine Cuellar, a spokeswoman for electric provider Oncor.

More than a half million homes and businesses—about one-sixth of the utility’s total customer base—were left in the dark during a series of storms that began Wednesday night and continued through Thursday. At the peak of the outages, about 260,000 were powerless at one time.

“What made this so difficult is the initial storm was followed by successive waves of lightning,” Cuellar said. “The lightning combined with tree debris in contact with our wires made it very difficult in our restoration.”

Records kept since 1996 put the amount of damage as the third-worst experienced by Oncor, she said.

Some residents said they were frustrated by what they believed was the slow pace of restoration.

“We have no air, we have thrown away hundreds and hundreds of dollars worth of food,” Yvonne Jefferson, 39, of DeSoto, said. “We have been without power since Thursday. Nobody is willing to help.”

Residents also were worried about neighbors who rely on oxygen or have other health problems.

“It is hot,” Eva Austin said from her darkened apartment complex. “It’s a mess.”

Repair crews from Houston, some 200 miles to the south, and from Louisiana and Oklahoma were working with Dallas-area utilities to restore power.