The Ukrainian authorities have warned people in the capital Kyiv to expect longer power cuts, lasting more than four hours, because of Russian attacks on energy infrastructure.
Rolling blackouts are hitting not only Kyiv but also central regions of Ukraine, including the city of Dnipro.
President Volodymyr Zelensky said about four million people were affected but "shelling will not break us". This month Russia launched dozens of missiles and Iranian-made drones.
Ukraine's energy infrastructure is being pounded by the air attacks - Mr Zelensky says about a third of the country's electric power stations have been destroyed.
The Kyiv region has lost 30% of its power capacity, the private energy company DTEK says, meaning "unprecedented" power cuts will be necessary.
The power cuts have meant curbs on the use of street lights and electric-powered public transport, besides the discomfort in people's homes.
The EU and other international allies of Kyiv have condemned the deliberate targeting of civilian infrastructure - attacks that Ukraine sees as war crimes.
Ukraine's second city Kharkiv, heavily damaged by Russian shelling, also faces long power cuts, along with the central cities of Zhytomyr, Poltava and Chernihiv.
Russia stepped up its missile attacks on Ukraine's power stations and other civilian infrastructure in retaliation for the 9 October bombing of the Kerch Bridge - a key link to Russian-annexed Crimea.