A UN-backed inquiry also found evidence that Russia recruited men from 29 countries into its armed forces "through deception or coercion" to fight in Ukraine in violation of international law.
The men "came mostly from vulnerable and marginalised communities, with a poor economic background and a low education level", it said.
The independent commission of inquiry was authorised by the Geneva-based Human Rights Council to investigate human rights violations shortly after Russia's full-scale invasion of Ukraine in February 2022.
Its report to the UN General Assembly's human rights committee on Monday was its first focused on the impact of Russia's attacks on Ukraine's energy infrastructure.
The electricity cuts have left millions of civilians suffering in frigid winter darkness, the elderly without elevators to get to shelters during missile and drone attacks, and hospital patients without critical devices to keep them alive, the report said.
The commission concluded that the attacks on Ukraine's energy infrastructure constituted war crimes because they were intentionally directed against civilian objects, as well as crimes against humanity for "inflicting great suffering to civilians", commission chair Erik Mose said at a UN news conference, speaking by video.
Mose said the intensifying attacks as this winter approached raise serious concerns about the threats posed to civilians.
The commission said it based its findings on the accounts of 255 Ukrainians who endured winter conditions in Kyiv and eight other regions in 2025-26 as well as government officials, doctors and civic organisations.
Russia's UN Mission said Russia did not recognise the mandate of the commission and claimed its findings were based on "falsified data".
Throughout the war, Russia has consistently maintained that its forces are only striking military or military-related targets in Ukraine, or targets that are used in the interest of the Ukrainian armed forces, despite multiple reports and evidence indicating otherwise.
Russian authorities have been recruiting foreigners into the army, offering perks such as accelerated Russian citizenship or making enlisting in the army mandatory for certain foreigners seeking permanent residency in Russia. Official rhetoric usually underscores the voluntary nature of such enlistments.
The report, however, said the men were recruited through deception and coercion in co-ordination with agents operating in other countries who presented themselves as legitimate and government-endorsed businesses.
The agents arranged visas and sometimes travelled with the men, who soon after arrival were forced to sign documents in Russian with the Defence Ministry that they didn't understand, the commission said.
"The recruits were shocked to discover that they would be serving for the Russian armed forces," it said.
The commission said it investigated the recruitment of nationals from 48 countries and found evidence that men from 29 countries were recruited into Russia's armed forces as a result of deception or coercion.
