A blackout has left millions of people without power in Havana and the rest of western Cuba in the latest outage on an island struggling with dwindling oil reserves and a crumbling electric grid. Government radio station Radio Rebelde quoted an energy official as saying that it could take at least 72 hours to restore operations at one of Cuba’s largest thermoelectric power plants, where a shutdown sparked the outage. The government’s electric utility said the outage affected people from the western town of Pinar del Rio to the central town of Camaguey. It is the second such outage to affect western Cuba in three months.