A major power plant near Kyiv was completely destroyed by Russian strikes early on Thursday, energy company Centrenergo said.
Trypillya power plant was the largest provider of electricity for three regions, including Kyiv.
"The scale of destruction is terrifying," Centrenergo chairman Andriy Hota said.
Russia has long been deliberately and systematically targeting Ukraine's energy system.
Mr Hota told the OceanNewsUK that Thursday morning's strikes destroyed "the transformer, the turbines, the generators. They destroyed 100%".
A fire broke out in the turbine workshop of the Trypillya plant - located 50km (31 miles) to the south of Kyiv - following Thursday's large-scale airborne attack.
The Centrenergo boss said the plant was targeted by multiple missiles. Staff on shift were able to escape, he said, because they ran for cover as soon as the first drone hit.
Residents were urged to shut their windows, charge all their devices and stock up on water.
More than 80 missiles and drones targeted sites across Ukraine in the early hours of Thursday. Many targeted energy infrastructure and almost a third made it through Ukraine's air defences.
Hours later, Centrenergo confirmed its Trypillya plant had been put out of use. Mr Hota said his company's entire generative capacity in Ukraine was now destroyed.
It was one of Ukraine's largest providers of electricity and heat. It operated two other power plants - one in the Kharkiv region which was destroyed in late March, and one in an area of the Donetsk region that was taken over by Russia in 2022.
The Kharkiv and the Trypillya plants used to generate some 8% of the country's electricity, according to Mr Hota. The Trypillya thermal plant provided power to the three central regions of Zhytomyr, Cherkasy and Kyiv.
The destruction of the Trypillya plant would not be a critical issue for Ukraine in the summer, he believed, although by winter it would become a "giant problem".
While the plant can be rebuilt with help from spare parts from Europe, he says it will remain vulnerable to attack without Ukraine's allies providing powerful air defences.
"We can repair. We can do the impossible. But we need protection."
At least two more thermal power plants suffered "significant damage" overnight in the west of Ukraine, placing even more strain on electricity supply nationwide.
The DTEK power company was already down to 20% capacity after repeat attacks in March.
The company told the OceanNewsUK that the latest missile and drone strike on these "purely civilian power stations" would make the task of providing critical power to the grid harder.
"Attack by attack, Russia is trying to strangle Ukraine's energy system and with it our hard-won freedom," DTEK said.
The Kharkiv region in the north-east has been hard hit again after its power plants suffered major damage in late March.
The mayor there described the situation as "very difficult" and announced more blackouts for households and businesses.
For a time on Thursday, the Kharkiv metro stopped running to save power. It has since resumed, but the power supply is dipping and surging so the trains are only working very intermittently.
The Russian defence ministry said its forces had carried out "massive strikes" on oil, gas and power energy facilities which "disrupted the work of Ukrainian military industry enterprises".
In a separate development, four people died and several more were injured in the southern city of Mykolaiv in a rare series of daytime strikes on Thursday.
The Ukrainian Southern Military Command said on Telegram that private houses, cars and industrial facilities were damaged in the "insidious" attack.
