DTEK said a bus carrying workers after a shift in the Dnipropetrovsk region had been targeted in Sunday's attack. At least seven people were injured.
Earlier, at least two others were killed and nine injured in separate Russian attacks overnight and on Sunday.
The victims included six people hurt when a drone hit a maternity hospital in Zaporizhzhia. Two were women giving birth at the time of the strike.
In a post on Telegram, Zaporizhzhia regional head Ivan Fedorov called it further "proof of a war directed against life".
He shared a video of smoke billowing from blown-out windows and photographs of shattered glass and burnt debris strewn inside hospital rooms.
Ukrainian Foreign Minister Andriy Sybiha said the attack showed Russian President Vladimir Putin was pursuing a "war against civilians contrary to peace efforts".
Fedorov later reported three people had been injured by a separate strike in a residential area.
After the strike on the DTEK bus in the city of Terenivka, the company said 15 miners had been killed. It later revised the death toll down to at least 12.
Elsewhere, a man and a woman were killed by a drone strike in the central city of Dnipro, Ganzha said.
Moscow launched a wave of targeted attacks on Ukraine's power grid in January, affecting heating and electricity supplies during an extraordinarily cold winter - with temperatures forecast to plunge below -20C in places this weekend.
US President Donald Trump had said on Thursday that Putin had agreed to halt attacks because of the cold - the Kremlin later said the pause would expire on Sunday.
Meanwhile, Ukrainian President Volodymyr Zelensky said a second round of three-way talks to end the fighting after nearly four years would begin on Wednesday, rather than Sunday as had been planned.
He did not give a reason for the delay, and said the talks between Russian, Ukrainian and US officials would take place in Abu Dhabi.