The entire system is down, the governor said. No one on the island has power from utilities.
Puerto Rico, which has been through a long recession and is deeply in debt, has a power grid that is "a little bit old, mishandled and weak," Rosselló told "Anderson Cooper 360?."
"It depends on the damage to the infrastructure," he said. "I'm afraid it's probably going to be severe. If it is ... we're looking at months as opposed to weeks or days."
The impact of the storm won't be realized until officials can do a flyover and see what remains. Rosselló said officials think some power stations are not badly damaged, but the distribution system is ruined. If transmission lines are in better shape than thought, power outages might be fixed sooner, the governor said.
Rosselló told CNN that at least one person died in the storm when a board was ripped from the house it had been nailed to by the wind and hit a man. The governor said the number of casualties in some areas is unknown because it is hard to communicate. "We still don't have a lot of information," he said. "We're virtually disconnected in terms of communications with the southeast part of the island."
Storm has moved toward the Dominican Republic
Now that Maria has moved past Puerto Rico, a US territory with 3.3 million people, search and rescue teams are taking to the country's darkened streets.
The devastating winds had died down to a whisper late Wednesday and the flooding rains were just a drizzle, but getting around was difficult due to widespread damage and no electricity except from generators.
The storm caused widespread flooding and ripped trees out of the ground. More than 10,000 remained in shelters Wednesday night.
"This is total devastation," said Carlos Mercader, a spokesman for Puerto Rico's governor. "Puerto Rico, in terms of the infrastructure, will not be the same. ... This is something of historic proportions."
A nightly curfew from 6 p.m. to 6 a.m. will take effect Wednesday evening and end Saturday morning, Puerto Rico Gov. announced.
Maria weakened to a Category 2 hurricane Wednesday afternoon, with winds of 110 mph, but is expected to gain strength with the core back over warm water.
Maria is next expected to pass by the Dominican Republic and the Turks and Caicos Islands, causing dangerous storm surge and rainfall. People in eastern parts of the Dominican were seeing whipping winds Wednesday night.
The once-major hurricane has already killed seven people on the Caribbean island nation of Dominica, said Gaston Browne, the Prime Minister of Antigua and Barbuda.
Browne said he had been communicating with the Prime Minister of Dominica, Roosevelt Skerrit, who reported "widespread devastation" and whose own house was shredded by the storm.