Power plant facilities in the central island of Leyte, which provide electricity for the island and neighbouring regions, sustained damage when the quake struck on Thursday, an energy department statement said.
The geothermal plants, near the quake's epicentre, were damaged by both the tremors and resulting landslides, the department said.
Leyte, home to some 1.75 million people, bore the brunt of the quake, recording two deaths and 72 injuries.
Much of Leyte and the surrounding islands of Samar and Bohol were without power on Saturday as repairs were still being conducted, energy undersecretary Wimpy Fuentabella said.
"In three to 10 days, we will see a tremendous improvement in ensuring that there will be basic electrical service available," he said on ABS-CBN television.
Vegetable seller Cheryl Anne Acidera, 25, in the Leyte city of Tacloban, recalled running out of her school in terror when the quake struck.
"We all ran out to the plaza so nothing would fall on us," she told AFP, adding that some of her companions fainted.
The Philippines lies on the so-called Ring of Fire, a vast Pacific Ocean region where many earthquakes and volcanic eruptions occur.
In February, a 6.5-magnitude quake killed eight people and left more than 250 injured outside the southern city of Surigao.
The following month a 5.9-magnitude tremor killed one person.
Before the Surigao disasters, the last fatal earthquake to hit the Southeast Asian nation was a 7.1-magnitude quake that left more than 220 people dead and destroyed historic churches in the central islands in October 2013.
