The Energy and Mines Ministry announced the restoration early on Tuesday, just hours after US President Donald Trump escalated his rhetoric against the communist-run island, saying he could do anything he wanted with Cuba.
Energy officials said they had reconnected the grid from westernmost Pinar del Rio province to Holguin, near the eastern tip of the island. Santiago de Cuba, the country's second-largest city, remained offline, the reports said.
State-owned media reported that by late Monday power had been restored to five per cent of residents in the capital, Havana, representing some 42,000 customers.
The Trump administration has made it clear it sees Cuba as the next country where the US can play out its desires on the world stage.
"Cuba right now is in very bad shape," Trump said, a day after Cuba's third nationwide blackout in four months.
"And we'll be doing something with Cuba very soon," the president said.
Until recently, Trump's comments on change in the socialist island nation might have been considered remarkable. But they come after his administration's audacious US military raid that captured then-President Nicolás Maduro in Venezuela, cutting off oil exports from that nation that had propped up the Cuban economy.
They also follow the launch of US military strikes against Iran on February 28.
The administration is looking for President Miguel Díaz-Canel to leave as the US continues negotiating with the Cuban government, according to a US official and a source with knowledge of talks between Washington and Havana. No detail has been offered about who the administration might like to see come to power.
Many Cubans don't believe that Díaz-Canel holds much power in Cuba, anyway, as opposed to revolutionary founding father Raúl Castro and his family.
Electricity was slowly being restored to hospitals and some homes on Tuesday afternoon, but officials warned that the crumbling power network could fail again.
The government blames its woes on a US energy blockade after Trump in January warned of tariffs on any country that sells or provides oil to Cuba.
Secretary of State Marco Rubio, who is of Cuban heritage, said the island "has an economy that doesn't work in a political and governmental system. They can't fix it".
A Cuban official said on Monday Cuba is open to trading with US companies, but such promises had been made before.
"So they have to change dramatically," Rubio said.
"What they announced yesterday is not dramatic enough. It's not going to fix it."
The Trump administration is also demanding that Cuba release political prisoners and move toward political and economic liberalisation in return for a lifting of sanctions. Trump has also raised the possibility of a "friendly takeover of Cuba".
While Cuba produces 40 per cent of its petroleum and has been generating its own power, it hasn't been sufficient to meet demand as its electric grid continues to crumble.
