In its statement on Friday, the authorities said power restoration has been slow because of the sheer scale and complexity of the damage. Much of the island’s 2,400 miles off transmission lines, 30,000 miles of distribution lines and 342 substations were damaged in the storm, they said. Carlos D. Torres, the system’s restoration coordinator, said workers were finding “unexpected damage” in some areas even as they make repairs in others.

José E. Sánchez, who heads the U.S. Army Corps of Engineers task force to restore power in Puerto Rico, said some homes were so damaged they are unable to receive electricity, though he did not know how many homes were in that situation.

The island’s governor and power utility said that of the homes that could receive electricity, roughly 55 percent have had their service restored.