Nearly 300,000 people were already without power by Wednesday afternoon, The New York Times reported. About 4,000 were without water because the electricity was out.
The Miami Herald reported that parts of the island could be without power for four to six months.
At 4 p.m. on Wednesday the eye of the storm was still about 70 miles northeast of San Juan, but the island was already feeling the storm's tropical forces.
The National Hurricane Center's forecast Wednesday afternoon predicted that the Category 5 storm would start pounding Puerto Rico within hours, passing by the island's northern coast.
Just after 3 p.m. local time, Puerto Rico Gov. Ricardo Rossello reported about 2,800 people and 230 pets were already taking refuge in shelters after evacuating.
REUTERS/Alvin Baez
Ricardo Ramos, director of the Puerto Rico's sole power company, PREPA, told local radio station NotiUno on Tuesday that some residents could have their power restored in as soon as a week, while for others it could take months.
Part of the problem is the island's debt crisis and crumbling infrastructure. PREPA, a government-owned utility, defaulted on nearly $9 billion in debt in July, NPR reported.
"The dangerousness of this event is like nothing we've ever seen," the Rossello said. "A lot of infrastructure won't be able to withstand this kind of force."
He urged residents in the path of the storm to evacuate, and President Donald Trump declared a federal emergency in Puerto Rico to free up federal resources.
The NHC predicted Irma would pass just north of Puerto Rico Wednesday night, and put the commonwealth under a hurricane warning.
Rainfall totals were forecasted to reach 4 to 10 inches, with isolated totals up to 14 inches. The storm surge - the quick rise in water caused by a hurricane's strong winds - was projected to reach 4 to 6 feet above ground on the northern coast, and 2 to 4 feet on the southern coast.
Hurricane Irma's maximum sustained winds were still 185 mph on Wednesday afternoon, and the NHC didn't expect them to slow down for days.
The storm is the strongest Atlantic hurricane ever recorded outside of the Gulf of Mexico or Caribbean Sea, and the extent of its damages are just beginning to emerge.
The #radar this afternoon of #Irma is absolutely textbook as it moves poleward of #PuertoRico's latitude. Eyewall should miss island. pic.twitter.com/A0qyVzStoU
- Philippe Papin (@pppapin) September 6, 2017