A recent wave of Russian missile and drone strikes have crippled almost half of Ukraine's energy system, Ukraine's prime minister has said.
The damage comes as temperatures drop below freezing and the capital Kyiv experiences its first winter snow.
One official in Kyiv warned that the city could face a "complete shutdown" of its power grid. President Volodymyr Zelensky earlier said that 10 million Ukrainians have been left without power.
Mr Zelensky said "the difficult situation with energy supply persists in a total of 17 regions and in the capital"
He added that there were "significantly fewer emergency shutdowns" during the day as energy industry workers were racing to restore power.
On Thursday, Russia hit more energy installations and civilian buildings less than two days after one of its heaviest bombardments yet.
This has been a recent Russian tactic following setbacks on the battlefield, and its impact is starting to be felt more acutely.
The government has urged people to save energy by reducing use of domestic appliances such as ovens, washing machines, electric kettles and irons.